Answer:
Some services allow you to white-label their software and profit from not having to spend thousands on development yourself.
After comparing the available white label text number services it looks like prices can vary greatly, with some charging as much as $9,000 for setup fees alone.
Number Text .com seems to have the lowest price point, charging wholesale rates on many of their services. They provide more details at http://www.numbertext.com/white-label-texting/
Of couse you can go to Jamster.com. But beware, when you download anything from them it "subscribes" your mobile number and they send more downloads whether you want them (or get them) or not.