A cheap option, if you have access to a reasonably stocked bike co-operative, is finding a cheap disc front wheel (mind was about $10), replacing the axle with a longer one, adding a few spacers, and using a bolt on cog. I sprung for the fancy factory made one by Velo-Solo, but I've known people who just drill cassette cogs out. Total cost for my fancy version was less than $50, and it could be done for far less.