I think painting the back of a tank is pretty inexpensive, and gives a really nice look. But that's best done before putting water in it. :/
As to the lights, why not just get the brooder reflectors from Home Depot/Lowes? I think they run something like ~$10.00 (smaller ones are less), and you can pick up some daylight CFLs for pretty cheap. You could spraypaint the outside black (or some other color), and even use the cords to hang them as pendants if you aren't going to be putting them right on the tank.
Also, with a 90 gallon tank, a flourescent shoplight will fit over it nicely, and that might give you more/cheaper bulb options (and at least less cords all over the place)
I'd avoid using acrylic to cover the tank, it warps/sags really easy (I've heard people say it's from the humidity or the heat-I don't know what it is, but I've had even small pieces warp significantly before)
I can't think of much else off the top of my head, you already have the tank, and that's a pretty big expense in of itself. I imagine stocking it will be the next most expensive thing. You could always understock on fish, and add more here and there (or let them populate the tank on their own). As to plants, if you have a local aquarium group, you could probably get quite a bit cheap/free at meetings. Otherwise, maybe go heavy with floaters (frogbit, water lettuce, etc.) to keep things in check while starting, and add a few scattered ones you like, and let them grow in and propagate.