Depends on what skimmer.
You'll find just about any place online will have cheaper prices than your local shops. If you're looking at name brand skimmers with decent reviews then I'd say you could be OK. If you're looking at those no-name skimmers that sell for like $20, then that's probably $20 down the drain. Skimmers don't have to be super expensive to get the job done, but it seems like the ones that are dirt cheap are a good example of 'you get what you pay for.'