27" wheels? Or are those 700c?

The cheapest way is just to get a rear wheel (maybe even Ebay). The less money on this bike the better.

The problem is you may end up spending plenty of money only to find out you hate the frame and have other problems (which lead to other money spending) Pretty soon you find out you could have almost bought a new bike or a barely used one off Craigslist.

I read plenty of threads of people complaining about their conversion. The dollar amounts can get high as well. (do a search)