Even ignoring the safety issues, it's really bad for a powerful device like a pump. I had my pump connected to a commercial grade extension cord and the voltage drop over 100 feet was enough that the pump's thermal protection would kick in as the motor started overheating in just a few seconds. Now you say your pump is a cheap one, so I doubt there's any thermal protection on it, and you could wind up melting the wires and shorting it out. Sure you GFCI and beaker box will probably do there job and protect you from an electrical fire that could burn down your house, but you'd still be out a pump for your pool, and would be risking a beautiful green swamp if you don't get a new pump fast enough.
If you don't want to take others advice and get a profession to do it, maybe you know someone with some electrical experience that could run some PVC conduit for you and hook up a proper electrical wire for you. With a question about the safety of an extension cord, I'm assuming you don't have electrical experience yourself. If you do try this yourself, please please please plug in a lamp or something and then throw the breaker to make sure you do throw the right one, don't want you electrocuting yourself over this.