What would be a reasonable cost to have a residential gas system pressure tested downstream from the gas meter. There is a gas furnace, a gas water heater and a gas range on the system.
The piping for residential pressure test shall stand a pressure of not less than three (3) pounds per square inch gauge pressure. The test pressure shall be held for a length of time satisfactory to the administrative authority, but in no case for less than ten (10) minutes with no drop in pressure, using a 15 lb gauge measuring in 1/10 psi increments.

Its impossible to give a price, as prices vary from area to area, but a standard test really shouldn't cost you too much as long as the test holds(no leaks are present). However, if the test does not hold, then finding the leak may get a bit pricey, as finding a leak can be a tedious job. Either way, having the test done professionally is really the only way to go. This is not something to take lightly. It can literally mean the difference between life and death. Good luck and please let us know how things go.