Sure the crabs themselves are cheap, but they have a start up costs in the triple digits($150 probably).
The 10 gallon(absolute minimum tank size) tank itself should run you around $10-$15.
I'm not sure how much a glass or plexi glass lid costs.
An under tank heater should run you $21. An 8 watt should heat the tank nicely. You can also try a heat lamp(I'm testing one out this weekend. ^-^ ).
http://www.petsmart.com/product/index.jsp?productId=2752652
Eco Earth is a very cheap substrate, one brick is $5 and should be sufficient enough to fill the tank(online it's cheaper). As much as I hate it, you can also try a good quality play sand from a home improvement store. In Texas, a 50 pound is around the same costs as an EE brick.
http://www.petsmart.com/product/index.jsp?productId=2752603
A 5-gallon bag of sea salt to make salt water(a MUST for hermit crabs) will costs between $4-$6. It'll last a while, so you don't really have to worry about that expense too much.
http://www.petsmart.com/product/index.jsp?productId=2754897
Smaller bottles of water conditioner is cheaper than this $4 one.
http://www.petco.com/product/105872/TetraAqua-AquaSafe-Water-Conditioner.aspx
This is a cheap thermometer and hydrogemeter, only $10.
http://www.petco.com/product/8193/PETCO-Thermometer-Humidity-Gauge-Combo-Pak.aspx
Shells for the crab are very cheap, go to a dollar store. If the dollar store is unreliable, go to Wal Mart or a craft store. It's $5 for a small bag of shells which is MUCH lower than most pet stores and other "hermit crab" vendors prices. 3 shells per crab. Painted shells should never be considered. They're expensive and can kill your crab when he eats the chipped off paint. Safe for humans does not equal safe for crabs. Don't try painting them in anything except safe to eat berry juices.
Buy three non metallic bowls for food, salt, and fresh water. One bowl is usually $5-$10.
The crabs themselves will costs $5-$11 dollars at a pet store.
You can go cheap on decorations. I've found a lot of nice plastic plants and plastic baskets to make a second level at the local dollar store(I used to buy shells there, but now they're all too small for my hermies. Hermies didn't get too big, the shells got smaller). Coco huts(hidey), coco walls(climbing wall), plastic plant(climbing toys), moss(good humidity increaser, snack, and digging area), pre-used fish/reptile caves(hideys and climbing toys), logs from the pet shop(both), ceramic unpainted pots(hideys), and legos(if you have any lying around, they can be hideys or climbing toys if you or someone you know is good with them) can decorate your tank quite nicely.For food, most of it comes from your local grocers and your local aquarium supply store/pet store. Commercial food is not an acceptable diet. They are scavengers in the wild and need a nice varied diet like we humans do. Fruits, veggies, meat, calcium foods(dairy in moderation/once a month, cuttle bone sold for pet birds, egg shell, honey), and other proteins(walnuts and such) should all be included in your hermies diet. This website is a big help with what they can and can't eat.
http://hermitcrabcuisine.com/
Save some money by not buying any "special hermit crab products". They're over priced crap. Sponges are a great example.
They're annoying because sooooooo many people believe they should use it in place of a water bowl. They won't drown(put a rock in the bowl if worried), and need to dip in order to get some water into there shells. They breed bacteria and need to be sterilized daily. All they are is a pain in the butt. Don't be fooled into buying them.
Your humidity should be between 75%-80%. Misting your tank can increase your humidity, but it's a pain to keep doing it. Make a little hold in the corner of your substrate, and add water. Moist substrate should keep your tank's humidity good. Moss pits are great for humidity too. Hiawatha moss is good for them and can be found at craft stores. Get a little plastic basket with good quality suction cups(if you're worried, aquarium silicone sealant is safe to use as a glue) and stick the moss there.
Start up costs are the real big expense, after that it should be smooth sailing money wise.
Edit:
Sorry, you just have no idea how many people who ask this question think hermit crabs can live happily on a $20 start up costs. Please don't listen to Linda.