I've done it before. I started stocking up on .380 before I even intended to own one, since I have a couple friends that had .380 pistols. Having a couple hundred rounds stashed was one of the deciding factors in buying my Bersa off one of them.
I also bought a $130 1080 round can of 5.45x39 with no intentions of getting an AK-74 in the foreseeable future. About 3 weeks later, someone made me an offer on my AMD-65 that I just couldn't turn down(I turned a $200 profit), and I went out and bought a
Tantal, which I absolutely love. I've put almost 3000 rounds of 53 grain surplus, and 70 grain Wolf through it since I got it in January without a single keyhole, malfunction, stovepipe, or any other issue other than the occasional light primer strike when attempting to bump fire.
My big "motivator" for going through with getting a gun is layaway. Aside from my mosins, every gun I've ever bought from a dealer was bought on layaway. I've never had a gun on layaway for over a week, even when I didn't intend to pick it up for a month or more. I figured out that once I put the money down and put a gun on layaway, it drives me nuts until I get it. It almost instantly goes from a passing idea, to the very top of the priorities list, second only to the absolute necessities.