How do I convince my parents a want a hermit crab?

So, I want a hermit crab. I can't convince my parents that I really want one. When I first said I wanted one, my Mom said I'd have to ask my Dad. When I did ask him he said our cat would do something to it.( it's in a cage ) Then I asked for one for Christmas, and they said you would have to buy a bunch of equiptment and stuff, and said that all of it would wind up spending 100 dollars( they have gotten me a lot of things OVER 100 dollars ). Then at dinner one night I asked if I could get a hermit crab if I payed for it. They complained about all the work it would be, and stuff like that. I said I'll take care of it completely and they still said no! They won't even tell me why I can't get one and I don't know how to turn it up. I don't see what is wrong with it, can anyone tell me why? Just letting you know, saying that I'll do chores and stuff isn't going to work, just tell me what you need for a hermit crab and why it's so hard to take care of.( I like people with personnel experience.)

Do you have ~100 dollars to spend on an appropriate habitat?

Hermit crabs are more difficult to keep than many people believe, and I highly recommend purchasing a book on their care before buying the crab. I personally bought this book and really feel that it contains all you need to know about hermits- http://www.amazon.com/Hermit-Crabs-Dummi...
Don't worry, I'm getting to the part where you convince your parents.

Once you have that book you'll be able to piece together a small binder of information about them. Include a page for every aspect about them such as diet, crabitat, basic needs, and even interesting facts. The last page could be a complete list of everything you would need including prices and the total. Presenting this to your parents shows that you have thought it out and done the research.

When I was little I had them all the time you need a 5 gallon tank a small heat mat you can use new paper for the bottom the spray bottle will be cheaper at the dollar store you need to get a few shells of sizes bigger then the one its in so when it grows it can move into bigger one don't get in trouble but be president

not helping

I have a pet hermit crab and have since I was five and still have one today. the trick is to ask them over time like you did and, show them how easy hermit crabs really are. they seem harder than they are. just spend some time with your hermit crab by feeding it by hand, holding it, and simply talking to it if you like. it is simple. My hermit crab Sapphire, is 10 this year. they can be long term. they can live 20 years! (trust me a cat wont hurt it.)