I'm not sure that there is a truly cost efficient way. As a rule of thumb a 9mm AR will run 10 to 20% more than a standard AR to build. To convert your lower to accept 9mm mags you'll need a converter which for a good one will run you close to 200, there are some for less but they don't work all that well. If your running a standard AR fire group the upper will require a enhanced bolt so that the fire control can be used, like has already be said it isn't difficult just takes money.