If a big sump is expensive, you might consider one of the smaller submersibles at HD/Lowe's. They have several models from which to choose and for about 60 bucks, you can get one that'll pump about 25 gpm or so. Just drop it in the bottom of the pool with a garden hose attached and it'll do the rest.
I bought one for that purpose and have probably used it 20 times for other projects....pretty handy device.
I bought one for that purpose and have probably used it 20 times for other projects....pretty handy device.