1. I would say bunnies (for me) are a 7. It probably depends on the person though.
I've had hamsters, fish, frogs, toads, and those were pretty easy... I've also had a chocolate lab, and becasue of his size (he can stand on his hind legs and reach the counters, and his nails scratch the heck out of our wood floors) and his activity level and his expenses (food, vet bills) I would rate him a 7.5.
I've also had a horse, and she was probably an 8 or 9 too becasue of the amount of time and money required for her care (I worked at the barn part time and had to up at 4 am to get to the barn by 5 every weekend morning ) and becasue I raised her from a baby and trained her by myself.
I think Leroy takes just as much time as a dog or horse. Also, bunnies are MISCHEVIOUS and because of their small size, they get into EVERYTHING. However, he's small and I can pick him up/nudge him out of the way if he's doing something bad. And litter boxes are easier than combing the yard for dog poop or mucking stalls... so I'd rate bunnies a 7.
2. As for expenses.... Um... $40-50? (Yearly vet check ups, at my bunny vet, are $50 + any additional meds or procedures)
I don't buy a lot of pet store toys since Leroy doesn't like them anyway - we do phone books and TP rolls and a cardboard tunnel for running through and the like. Also, I save money by buying hay in bulk - I'm about halfway though a square bale of hay that I got back at the end of August (free from a good friend! bales generally run $5-15). I use wood stove pellets ($5 for a 40lb bag that lasts me ~5 months) and newspaper (next to free) for litter. Veggies and pellets are what's the most expensive. I don't skimp on the veggies - he gets about 2-3 cups per day of 3-5 different veggies, and I buy them weekly from the grocery store ($7-10). A bag of Kaytee pellets (5lb) is ~$13 and lasts me 3 ish months - I haven't found anywhere to buy good quality pellets in bulk yet!
That's not including the initial cost of the rabbit itself and spay/neuter (if you adopt, most shelters desex them before letting them go home! so that's a huge money saver since spaying/neutering can be upwards of $200) and the start up costs of a cage and bunny proofing supplies and a boatload of toys
You can definitely find ways to spend more $ though!
3. Well my first bit of advice would be to do your research and check out a good informative website, like BB or rabbit.org. But you've already done that! I think the big thing for you is to consider how much time you have to devote to a rabbit? You've mentioned that you have other pets, so a rabbit might be adding too much.
I'm in college right now so my schedule is pretty flexible and I can spend a lot of time at home, letting Leroy play. Bunnies need companionship and also a few hours of play time out of the cage per day. Leroy has to stay in his cage when Im not home or when I'm asleep becasue he's naughty But he gets ~8+ hours out of his cage... and even though I've bunny proofed, for most of that time I have to be near by to keep him from getting into stuff.
I know how busy high school is and it's only going to get busier with college apps (if you choose to go that route) and such as you get closer to graduation. Also. keep in mind that dorms don't allow pets and most colleges require that you stay in the dorms at least one year. Are your parents willing to take care of the rabbit properly when you're in school?
Also, as with any pet, there can be HUGE unexpected expenses - ie. vet bills and as a student or a young adult trying to find a job just ouf of high school, if you have no savings, you're going to run into major problems.
Oh, and keep in mind that, like any pet, you'll need to care for them regularly... meaning you have to be home every day to care for them, and you have to either bring them or arrange a bunny sitter for vacations and the like.
I'm not trying to talk you out of a rabbit or to scare you, but... rabbits are a lot of work. I think its GREAT that you're doing your research and trying to make an informed decision.
Hope that helps!