As svdorr said check out the competition, but don't look at just the cheap ones, look at some long term players who have made it a few years.
Also once you have some numbers make sure it works with your business plan. A good business plan can be very important is starting up a new company. Not everyone does, but it well likely help longterm.
You may also want to do a search on the forum for this topic, it has come up before, in the reseller hosting section as well as in running a webhosting business.
PS: Moved Thread to Running a WebHosting Business as I felt it fit better there.