Calculate the gearing on your MTB and print out a chart (I use gear inches, the SheldonBrown.com gear calculator is handy to get a printout from). Mark the chart for which gear ratios work for your commute, mark which gears are too tall for hills, which ratios are too low for the flats, which are comfortable, which are not. Seems most folks end up with SS gearing in the 76" range +/-6" or so. Also, if whatever bikeframe you end up with has horizontal dropouts you might be able to go with a flipflip hub on back and have two gear ratios to pick from, one for daily use and the other for windy or tired days. If your frame ends up with modern dropouts you might be able to use something like a singulator for chain slop tension.

Cost will depend on new or used, your mechanical capability, how good or lucky you are finding deals, and on what you'd be happy with. You can find a used bmx bike to salvage a freewheel from, and several rummage sale or thrift shop bikes to accumulate more parts from. One I set up I had maybe $15 into total cost. My Lemond Fillmore was bought new and cost quite a bit more even with a seasonal employee shop discount.

Set your budget, figure out your gearing, decide new or used or mixed, check your local market for deals, and go for it. I like having a singlespeed available, but I also like having a geared bike too. N+1 is fine if you have the budget and room.