There are two parts to your question. The first part is about the cost of conversion. The second part is about the fuelling station set-up costs.
I'll give you a range for the unit cost for the cost of conversion for propane. With more volume, of course, the cost will drop as you take advantage of economies of scale, but the cost of conversion ranges between $3,000 and $6,000, approximately, and it's between $45,000 and $55,000 to set up a fuelling station.