Hi Ed!
The cheapest way - but by all means not what a proper single speed will look like - is to pick a front ring if you got more than one (I would pick the middle one or the big one), shift the chain, take the front derallieur, cable and shifter off the bike.
Then pick a back sprocket to put the chain on, set the hight/low screws so that the chain won't shift, but leave back derailleur, cable and handlebar shifter on the bike, for chain tension.
Cost: zero
Look of the bike: .... ermm
![Wacko :wacko: :wacko:](/styles/default/xenforo/smls/wacko.gif)
![Laugh :laugh: :laugh:](/styles/default/xenforo/smls/laugh.gif)
but it works.
You might want to experiment riding in a few gear combinations before you make the final modifications to the bike.
More mechanical savvy folks will come along shortly for sure
![Wave :hello: :hello:](/styles/default/xenforo/smls/wave.gif)