There's more to Meridian speakers than $50 tweeters - think about the technology in the 5500's - 4 drivers with an amp driving each, a built in pre-amp and DAC's - heavy duty cabs and a lighted display - all of it selected, R&D to work together in a cohesive presentation. You have to also build in manufacturer and dealer profit otherwise they would cease to exist - what it cost to also manufacture the parts? Seas is able to sell the tweeters at a reasonable price thru economies of scale.
Another big point is because of the internets ability to share information we are aware of replacements and issues that effect our gear - this info was not available for mass consumption when my 5500's were first built.