Some expensive diplexers have a sharper roll off between bands, which makes a difference if you are using them in Europe where the UHF broadcast band goes up to about 860Mz. Some expensive diplexers have a slightly out-of-band rejection factor, which can make a small difference if you decide to try to backfeed your channel 3/4 outlet onto your satellite input line. I advise against that.
For 99% or more of all real applications, there is no benefit to using a so-called "premium" diplexer, and price is not a reliable indication of performance. Many $20 diplexers perform the same as $2 diplexers.
Neither of these characteristics matter to you. Radio Shack used to sell dual LNBs for $200, and they weren;t any better than the less expensive ones, and their $99 3x4 multiswitch is no better than the ones you can buy on the internet for under $30. Shack gives away the glamor item and then tries to make a substantial profit on the accessory that the customer is less likely to have price shopped for.