If you have already applied and received your $40 dtv coupon, you can use it with the purchase of your converter box at a particpating retailer that can accept the card. At Walmart or Best Buy, you can expect to spend about $10-$25 total after the coupon discount has been applied. One of our forum members has reported that a particular store is selling converter boxes for the same price as the coupon at: http://www.dtvusaforum.com/everything-else/768-rite-aid-sells-converter-boxes-40-a.html . At that price and with a DTV coupon, you'd only pay sales tax on the converter box.
If you do not have a coupon, you can expect to spend anywhere from $45-$55 on a converter box only meant for converting your television to accept dtv signals. There are some converter boxes with extra options such as Digital Video Recorder, or High Definition output, on the market that retail anywhere from $75.00 to $250. If all you want to do is upgrade your TV for the digital transition, the $45-$55 models will be just fine.