I think a tent is totally worth it for an indoor grow.... However don't cheep out when buying one. Many of the cheep Chinese ebay ones don't use mylar and instead use some kind of toxic plastic that gives off gas and harms your plants. I have a 4X4 Lighthouse tent (from the makers of the blackstar LED). They have other sizes. Not cheep but not expensive... The main reasons I chose this one is it has a metal frame, it has pouches on the sides to hold meters, clippers, cords, etc, and it has a fitted tarp to go over the floor that has a 4 inch lip. That way if there are any spills or leaks the tent will contain them and they wont ruin the carpet in the room.
http://www.ebay.com/itm/4-x4-x7-Myl...442?pt=LH_DefaultDomain_0&hash=item3f0cd87eaa