I'll sell you a national level 450 racer for about $5k, with all the SM and MX stuff included.
To build one, you've gotta start with an MX bike platform, so say, $3500 for a used bike.
Add:
wheels: $1000
Brakes: $250-$500
now you're at $5k for an MX bike with 17in wheels, which isn't REALLY a supermoto bike.
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