Prices? About?

How much for 4 5-7 spoke rims and tires for a 91 DX Civic...? What size are they?

just depends on the size, color, wheel manufacaturer, tire maker. there's sever sites that have wheel/tire packages available.
www.tirerack.com
www.wheelmax.com
www.victoriatire.com

one thing to keep in mind though, you mostly get what you pay for.

Could I get some good LOOKING rims and fairly low profile tires for under 750 US?

Yes, you can get whatever you are willing to pay for, but remember this...

1. Light weight wheels
2. Strong Wheels
3. Inexpensive wheels

You can have any two, but not all three.

You should really look at those sites that were recommended.