The Mustang and Rotary guys have been running methanol injection for years on S/C and turbo setups - and none of the kits cost a lot. Snow is an excellent system. I have Anderson's Mr. Freeze kit on my 5.0. If you look on Ebay, you can find Mr. Freeze kits around $100 at times. Mine was less than that. If you want really cheap injection, you can go to Ace hardware for windshield washer fluid. The stuff they sell at auto parts stores is different here in the south, so it doesn't have the same methanol content. Ace only sells 1 type across the nation.
The AEM can control the injection, but with the Mr. Freeze kit, it only pumps the amount your jet is set for, and only under boost, so it's not necessary. There are no pumps to have break on you, so your engine is safer. You use the output from the AEM to control your nitrous. This way you can have the best of all worlds.