400 hours is nothing for an Onan generator. More commonly generator problems come form a lack of use than from wearing out due to overuse (and that's much more of a problem with gasoline powered generators than with propane generators, as propane doesn't tend to turn into varnish in the carburetor when left standing for a long period of time).
Replacement cost would generally be in the category of "if you have to ask, you don't want to know." (I would guess four figures...but hesitate to guess which four.) I'm with gbopp and the others that it's not necessary to even think about replacement now; if it runs well, it should continue to do so without much drama for a good while, possibly with one or two fairly minor repairs.