what decent phone can I buy that would allow me to use the N800 for internet service that has a low monthly price??

Very cheap phones in Eastern Europe, Indian sub continent and most of Africa. Go on, give us a clue where you are and don't make us assume you're an American (this is a global forum, ya know).

I'm from nyc

Any Nokia or Sony Ericsson phone with Bluetooth will work with the tablets.

Next, take a look at your local cellular carriers in NYC - their web sites are usually a good place to start - and see what their monthly plans are like, and choose the cheapest that offers a Nokia/SE phone.

can it be any phone with bluetooth

Avoid Windows Mobile smartphones as they don't support Bluetooth DUN (Dial Up Networking). Samsung phones should be OK. In fact, any phone that supports Bluetooth DUN *should* be OK, but I would only say for sure that Nokia and SE phones work without issues on the whole... there have been reports of problems with some Samsung phones, maybe these were WM phones (do Samsung make WM phones?) Motorola I have no idea about, as I tend to avoid them

My wm5 htc ppc-6700 on sprint evdo ($15/mon unlimited) works good as long as you dont move around much while connected. Some times the phone crashes when trying to stream large videos.
but just for email/rss, chatting and casual browseing it works good.
i've yet to try the hacked roms. i've read they are better than the offical.

I have a Razr using Sprint's unlimited PAM (phone as modem) service at 39.95/month. It works GREAT!

I'm looking for another phone. Do you disconnect when moving around? Does that 39.95 include voice calls?

That monthly fee probably doesn't include voice calls. And yes Samsung makes wm phones, i607/Blackjack for instance. Which doesn't work with the bluetooth pairing, btw.