Unfortunately if you have to convince your mom to let you have one, you probably shouldn't get one. How old are you? If you're planning on going to college in the next few years- who will care for your hedgehog? Most dorms don't allow them and a lot of apartments won't either. As well, once your life changes you may not have the time to care for your hedgie anymore and that's not fair to the hedgie. You should always have provisions for him/her and for any major vet bills that arise....I know you said if you had to, you would give him/her back, but just remember how hard it can be to adjust to change and how unfair that is to your animal. Another thing to consider is what you will do with him/her if you go on holiday? Someone will have to babysit and it will need to be someone hedgehog savvy. If your parents don't want you to get one, they may not be interested in taking care of him/her if you go away for the weekend or on a trip, and then you will have to find a babysitter.
Also to give you a bit of perspective on costs, from someone who had no idea how expensive a hedgehog can be: I had an issue with my hedgehog not eating properly just weeks after I got her, and paid over $500.00 at the vet (in 3 visits) and there was no emergency care or hospital stay involved (those make costs go waaayyyy up). As well there are the initial costs - with her cage, toys, food, heater, liners etc it all came to somewhere in the 400-600 range. So in that first month and a half we had her I spent over $1000.00 on her. I'm not saying that's normal, but I had not prepared for that, so as you can imagine I was very shocked. Thankfully I had resources I could use in this situation and I am really glad I did have them. If you don't and you're parents aren't on board with it, you would have to return him/her in a similar situation and that would be unfortunate for everyone involved.
So definitely waiting a few years (like you mentioned earlier) sounds a good idea, especially if you plan to go away to school.