Cheapest way is to buy yourself an IFR C-172, or even a C150/152 and fly the snott out of it getting your ticket and instrument rating... you can even do the commercial in it, but for the checkride you'll have to go back, swap into something like an arrow and demonstrate the complex portions of the exam... still way way cheaper than renting at a flight school, so long as you go after the tickets with resolve, get it done, and then sell the plane. You'll be out the insurance costs, tiedown fees, and fuel costs. If you do it all quickly enough you won't even have to pop for an annual.

That is the cheapest way....