Don't even think about this. Airlines are always on the look out for hidden city ticketing and attempting to short check your bags is the clearest sign that you're trying it on. Here's a case of someone who got caught and had to pay up -
If (let's make that when) you get caught, what are you going to do? Carry on all the way to Brussels to get your bag and pay for another flight home? Or settle with the airline? And don't think settling with the airline is going to be cheap. You'll have to pay the change fee and difference in fare. And the fare difference will be between what you originally pay and a same day walk up one way full fare - which will make your eyes water - not the difference you're currently seeing.
The reason MAN-LAS costs more (I''m assuming this is Virgin) is because it's non-stop. If you want cheaper fares then look at indirect flights. You have BA via Heathrow, and also all four of the main US carriers fly to MAN from either JFK, Newark, Philadelphia, Atlanta, Chicago and Washington-Dulles.