First of all, it is important to be able to determine what kind of a project it is by way of budget. When it's a small indie company putting a shout-out for volunteers, you can generally pick up on the fact that they are digging for loose change under the couch cushions just to get the project done. But once in a while you will see advertisements for productions who claim to have big name actors or being funded by known networks begging for volunteers to fill crew positions, and that rightfully should raise an eyebrow. I'm not saying that volunteering to do such a gig will hurt, I'm just saying that you should go into it knowing that you will be taken advantage of and weigh the pros and cons for yourself.
However, for the purposes of this essay, I'm going to assume that the project isn't listed as a volunteer gig but also isn't tied to any multi-million dollar networks or has a lot of star power behind it. So the trick is making sure that all your needs are met without giving them a quote they really can't afford.
The first thing you need to consider is how much your materials are going to cost and how much time you are going to spend preparing everything. Keep in mind that most people don't understand how expensive makeup supplies can be or how much work goes into things like prosthetics and FX gags. (Most women don't spend an upwards of $3,000 on their makeup kits, which is usually the only thing people have to compare this to. You sometimes have to remind them that most women don't have to be able to match more than one skin tone or apply prosthetics.)
For out-of-kit makeups, many artists just charge a flat kit fee (in addition to their salary) to cover the anticipated expense of replacing the supplies they use from their makeup kit. This is generally determined by how large the job is and doing makeup for a couple characters will obviously need less of a kit fee than doing a dozen or more people every day.
But for FX makeups and prosthetics, it's a completely different story. It takes both a lot of preparation time and money to take the life-casts of the actors, sculpt and prepare the appliances, and often requires more than one copy of the prosthetic to be poured for multiple-day characters.
I invest in myself by buying many of my commonly-needed prosthetic supplies in bulk, which allows me to charge less for materials than if I bought only what I needed for each production. (I wouldn't recommend doing this unless you do take a lot of gigs, as the shelf-life on materials like alginate isn't very long.) However, it's good to shop around and get prices on supplies from various resources so you have a general idea of what each project is going to need. Also be sure to allow yourself a contingency for those unexpected expenses.
Once you've got an idea of material costs and pre-production time you're going to spend, it's time to consider other factors.
If you live in a city where many productions shoot, you probably won't have to consider your travel expenses. But if you are like me and live in a small town far away from the film-making hotspots, your gas and hotel charges can really add up quickly.
Notice that I mentioned hotel charges. It's funny how many people ask me why I favor staying at hotels rather than drive back and forth when I'm working on a project that's less than 100 miles from home. I do it because it's actually less expensive to get a room than to gas up a fully-loaded SUV for the two-way trip every day. (Not to mention less wear and tear on my car and reducing the chance of getting caught in traffic or getting a flat tire when time is an issue.)
For projects that are going to need you on set for at least a week, it's worth looking into vacation rentals or extended stay hotels. These places generally come with fully-equipped kitchens and more space to spread out for a lower rate per-day than a standard nightly-rate hotel room.
Also, don't make the mistake of thinking that because the production offers meals that you don't need to consider the expense of food and drink. Unless you can go a full day of working hard with only one meal in your belly, you're probably going to want a little extra to swing by a drive-thru while running around and missing the on-location chow time. (This is also where having a kitchen in your hotel comes in handy.)
The last thing to consider, and this is often the hardest for people to figure out, is what monetary profit you want to see for your time and effort. While it's easy to just come up with a day rate based on industry standards, many low-budget productions will shy away from a flat $300/day on a project that runs longer than a day or two. You either have to hold out for projects that can afford it, or negotiate for a salary that's comfortable for both parties.
I'm not a big fan of charging flat day rates for multiple-day projects. I've been on plenty of productions where I've had many "light work" days and then one or two heavy FX days on set. I can't see charging a production $300 daily for my two-hour days as well as my ten-hour days. Maybe things are different in Hollywood.
As it is with most crew positions, the more equipment and skills you have, the more your street value goes up. A DP with a Red One can usually get a lot more per day than one with a consumer-grade camcorder. However, I have noticed that some people will over-play that hand when it comes to negotiating salary with low-budget productions. While you may be trying to pay off the credit bills on your equipment (or loans for school), you also have to decide whether or not to go with many projects at a lower rate or hold out for one or two top-dollar projects to clear your debt. You may have a long wait if you go with the latter option.
In my experience, most low-budget productions really do wish they had the money to pay their crew what they believe they are really worth. They might offer you a small rate up front and points on the back end. (Although those points are likely worthless given the success rate of small productions, this should never be taken as an insult because film-makers do believe in the value of their projects and sincerely expect them to succeed.)
Very few productions will find it unreasonable if you ask for a rate to at least cover your materials and travel expenses and they will let you know up front if they can't even afford that. At that point it's up to you to decide whether or not to volunteer.