So uhh, I ended up going
kinda deep down the rathole on this one. I've known about
gpmotorbikes.com for a while; they've usually got a good mix of Moto3, Moto2 and the occasional MotoGP bike for sale. They don't seem to list prices anymore, which is annoying. From memory, a Moto3 bike would be about €10,000-20,000, and Moto2 bikes higher again. I can't remember what MotoGP bikes would go for, but it's considerably more again.
raresportbikes.com is an index of rare and not-rare-but-interesting bikes for sale; it has descriptions of stuff and all the bikes are on eBay. It's mostly road bikes, but you occasionally stumble across something interesting, like
this 1989 Cagiva V4 500cc GP bike that went for a cool $55,000.
There's also
RMD Motors in Japan, though they're more focused on older stuff. The newest they have for GP bikes is around 2002.
Where things got really interesting was the innocuous-sounding
FIM CEV Repsol teams community site. They have listings for everything from
2015 KTM Moto3 bikes to
teams looking for riders (!) and more. You can even pick up Tito Rabat's old
CBR600RR training bike for around €8,500.
If it were me, I'd buy
this NSF250R Moto3 bike from the BSB Moto3 series (I didn't know that was a thing?) that's got a zillion upgrades for €13,500. It's cheaper without the data acquisition stuff, save yourself some money! Seriously, that's about $15k in real money.
Last but not least, if you want a full-on classifieds experience, there is the aptly-named
Race Bike Mart. There's a lot of parts and rubbish in there, but occasionally you find something like this
1993 YZR500 factory for a cool €150,000.
xxaarraa, you wanna go into business flipping this stuff?