If you are talking about buying extract in bulk then you would differently save money, obviously.
Extract is concentrated, flavored sugar. Adding more extract (say 5 lbs) vs same ratio of grain (say 5 lbs) would equal sweeter beer and higher alcohol content. Like runningweird says, you would typically need more hops to balance the sweetness. And I agree with him also in terms of looking up the sugar content of extract vs grain.
I would suggest using a beer recipe program (Beersmith, etc.) to determine the difference. Many of these programs will show you how balanced your sweetness and hoppiness is with the style you are shooting for.
As a former extract brewer who is now all-grain, I would suggest making a switch to the brew in a bag method or a cooler all-grain setup. It costs less with only slightly more work-plus the beers seem better.