The main thing to remember when staining concrete is it has to be clean. You need to etch the floor with a mixture of Muriatic acid and water 3 to 1 ratio (water to acid), then hose off and let dry. Concrete stain ranges from about $20-$30 per gallon. Dilute the first coat about 50% then the second coat full strength. Hope this helps.
Wendy, as with many projects, the real answer is ... it depends. Is the floor covered with carpet, linolium or tile currently? Will you remove or will a contractor, how many square feet will be stained? Are you looking to do it yourself or hire a pro? It can run from pennies to many dollars per square foot.