Start up costs are around $150, but after that they aren't expensive.
The 10 gallon(absolute minimum tank size) tank itself should run you around $10-$15. If you don't want to spend money on a the tank, you can always go to Craigslists and see if you can pick one up for free. Garage sales are another good place to find them for cheap.
An under tank heater should run you $21. An 8 watt will heat a 10 gallon tank sufficiently. A heat bulb around 35 watts will heat the tank as well. You can pick one up at a home department store for cheap. If that bulb wattage will not heat the tank sufficiently, you can try going up the size just above. Lighting might have a bad effect on your humidity if you get a ridiculously high wattage. This is a link for an under tank heater.
Eco Earth is a very cheap substrate, one brick is $5 and should be sufficient enough to fill the tank(online it's cheaper). Expand it in pre-made salt water to help prevent mold, moisten as needed(poke a hole in the corner and gently add water). This is a good EE deal.
A bag of play is another cheap substrate that you can use. One 50 pound bag is usually $4-5. This can be moist or dry. What ever you prefer. Fine coral sand and aragonite sand are good options too. They only draw back when compared to play sand and EE is that it's more expensive.
You will need aquarium grade sea salt for your hermies(yes ALL hermies should have access to salt water). You don't need to buy a large tub, a 5 gallon bag such as the one shown in the link just below will last a while.
Here is a link on how to properly mix salt depending on brand.
A simple hydrometer can come in handy, but it's not required.
Unless you use distilled water, you're going to need a dechlorinator. As long as it doesn't have stress or slime coat it will be fine. I use this Top Fin.
A dial or digital thermometer and hygrometer(or a combo of the two) will be needed to measure the temperature and humidity. Any home improvement store should have a cheap combo with not metal attachments. The dial combo in this link will do the job as well, but calibrate your hygrometer as needed(here's a link of it
You do need a container for your food. You can use anything like a peanut butter cap or clam shell. As long as it's not metal. For the water you're going to have to find something that allows your crabs to add water into the shells easily(or bathe basically). I use Tupperware for salt water(it's a large pool)and a reptile dish for fresh. Mini Tupperwares can be used as a bowl.
The crabs themselves will costs $5-$11 dollars at a pet store. Remember to look around for a good one.
You can decorate your tank with just about anything. Most all my decor comes from a dollar store(the used fish cave is probably the only thing that doesn't). Here's a list of some stuff you can use for decor(from my head mostly).
coco huts(you can make them yourself or buy them)-hidey
legos(you can make some nice things if you're good with them)-hidey and climbing
A shower caddy makes a good second level
moss pit(oodles of fun for the burrowing hermie, but never use it as a substrate)-humidity
log(none collected outside please)-hidey and climbing
a plastic basket-climbing
reptile/fish caves(some hamsters hideys like igloos are fine as well
a rock wall-climbing
organic bromeliads(WARNING-they probably won't last long) The hermies will probably destroy them quickly. Anyway it's a nice treat and climbing toy(if it's big enough). :3
plastic containers(if you got a tall one, you could fiddle with it to make a hidey/climbing toy)
Strong suction cups(maybe some sold for fish tanks, weak ones don't hold up well) and aquarium silicone will hold just about anything in your tank.
Your hermit crabs should be given a varied diet. They shouldn't be forced to eat those little bottles of commercial food. Meats(seafood, a bit of occasional beef, insects and other inverts,etc...), veggies(broccoli, carrots, spinach, etc...), and fruits should all be included in their diet. This website will give you a good idea on what they can eat and some basic food groups. Your crabs might not try something the first time, don't worry. Just be patient, and they should try the food on their own. You might be able to coax them if you mix the new foods with an old favorite.
Shells are rather cheap at Michales or some other craft store. A bag of good ones will only cost $5. NOTHING painted or messed with like that glow-in-the-dark shell. They aren't good and will harm your crab. At least three shells per crab(preferably more) and they shouldn't fight and go naked(at least for the lack of good shells reason).
This link is pretty good for safe ways to cut costs.