Sorry for long post, but you asked for opinions and advice XD
If you want contacts to change the colour of very dark brown eyes to a lighter brown you will need the special "coloured opaque" lenses. Even if you don't need specs or lenses to correct your vision, you'll still need to get coloured contacts prescribed by an Optomotrist (it's the law in a lot of Western countries).
So even tho' ppl will post replies here, the most important thing is to get professional advice about it before you make any decisions.
Coloured opaque lenses are good at changing your eye colour but there are downsides. Even though there are different sizes to fit different eyeball shapes there will always be times when the coloured part will slide over your pupil (the black bit in the middle of your eye) - they always move slightly when you blink - and everyone blinks regularly (contact lens wearers more often than people who don't wear lenses). This can be quite irritating and obstruct your vision a little.
Also, the size of your pupil changes all the time (to adjust to different light conditions) — so sometimes, like in the evenings when it is much darker or when you go inside from being outside or when you move from room to room, your pupil may be larger than the clear central part of the lens that you are using to see through. If that happens, your vision can be slightly affected. If you drive a car - or intend to get a licence to drive, you might find that you will not / should not drive at night in the coloured lenses if your vision is affected.
If you wear make up, contact lenses can be irritated by bits of eye liner, mascara, powdered eye shadow, etc. and make your eyes water which also makes the contacts uncomfortable to wear - watery eyes also totally stuff up your eye make up!
What it comes down to is this. You need to go to an optomotrist and talk about the options and prices in order to make an informed decision. You shouldn't have to committ to buying contacts with a first visit - after all, you are checking out what's available, what's right for you, what the pros and cons are and how much it is going to cost you.
As for cost, it depends if you are earning yourself, if you can afford it and are prepared to spend the money on lenses instead of clothes or games or DVDs, etc. If you are hoping your parents will pay, then it is even more important to get the facts from the optomotrist on how suitable your choice of lens is for your eye and how much your parents will be expected to pay
Finally... I also have very dark brown eyes - they almost always come out looking black in photos and no pupil visible. I also wear contacts (to correct my vision only - not to change the colour). I know so many people who wish they had a strong eye colour like mine and are envious of my eye colour. I don't know many people who want to make a rich dark brown more pale
Good luck
