Lowering 2001

What is the best way to lower a 2001 sentra, and how much would it cost? Thanks

The best way is a set of full coilovers, but it's also the most expensive way. You have a few choices:

1 - springs - these will give you a decent drop, but ride height in the front will not match the rear. Your stock dampers will wear out prematurely and you'll need to replace them, as of right now the only aftermarket dampers are the Koni Yellows, which reuse the stock housing in the front and also cost over $500, and since they reuse the stock housing I don't believe you will gain any wheel travel in the front.
Initial cost ~$150
Final cost ~$700
no ride height adjustability
no added travel in the front

2 - half-ass coilovers - these are the ground control coilovers and most of the ones you see on ebay. They'll give you ride height adjustability, but they reuse the stock dampers. Like the springs, eventually your dampers will die and you'll need to go aftermarket. Again the only choice is Koni, and you have the same problems that you had above.
Initial cost ~$300
Final cost ~$850
plenty of ride height adjustability
no added travel in the front

3 - full coilovers - these are like the half coilovers above, except they include their own dampers that are built for the coilovers and have shortened dampers for more wheel travel. They're the best option but the prices range anywhere from $800-$1800. The cheapest ones are the Tein Basics, they'll give you as much ride height adjustability as you want, but they don't have adjustable dampers. If you don't race then this shouldn't be a problem though, I have the basics and they're very nice. More money buys you more damping adjustability, the Tein SS run around $1100 and the JIC A2s run around $1800. But if you don't autocross then I don't think there's really a point in stepping up to these ones.
Initial cost $800-1800
Final cost same
plenty of ride height adjustability
plenty of added travel in the front

I think another big reason for gettin the Tein SS coilovers is the EFDC option, cockpit mounted adjusters, kinda neat, But I would recommend the Tein Basic, you will feel a really big difference.

yeah the EFDC would be nice

also the springs on the tein ss aren't as stiff as on the basics (325 vs 375 I believe), which isn't exactly a bad thing if you have a sensitive ass ;) On a rough road the basics can be pretty brutal, but when the car's dropped over 2" all around it needs to be stiff to keep from bottoming out.