OTR Over The Range aka cook burner stove top
just goto home depot and buy the one you want
the "width" is standard, the Height and the depth can vary a little
when our over the stove factory installed GE MW died
we went to home depot and bought another GE over the stove aka UNDER the Cabinet
MW and installed it in the same spot
it will come with its own mounting bracket, and a template for marking/making aligning the bracket and holes
and NO it won't be $500 unless you buy the convection oven model
straight MW $300 or less,
i think the guy who told you that story is confused
he probably bought a "trim kit" for the MW he was installing in a cabinet
not an empty MW shell
trim kits are for installing 'counter top' MW's inside a cabinet
NOT for hanging one over the range/cook top
if you want a shell
take yours down, take it apart, GUT IT,
hang empty shell with the door
buy new smaller MW and place inside of old shell