I don't know if you're interested in TP twins, but you can run a Garrett powered twin for just about the same money as you run any PT6 single or a 421C, all in. A Turbo Commander, MU-2, Conquest, B100 King Air etc. They burn 20-30% less than PT6's, cost about half as much to overhaul spread over the 5000/5400hr TBO, go faster and further and have more payload. But perhaps even more surprising is that they will cost as much to operate as a legacy twin like the 421C. Let's do a numbers comparison that I stole somewhere:
421C
GTSIO-520 overhaul: $70K/1600 hours
Exhaust: $10K/1600 hours
Oil changes: $250/50 hours
Magneto work: $1500/500 hours
Top overhaul: $15K/1600 hours (one per TBO)
Fuel: 22 GPH block/hr, $5.50/gal
Total works out to $68/hour without fuel, $188/hour with fuel.
MU-2/Turbo Commander/B100:
TPE331-10 overhaul: $225K/5000/5400 hours
Exhaust: none
Oil changes: $400/900 hours
Fuel nozzles: $800/400 hours
HSI: $40K/5000 hours (one per TBO)
Fuel: 37 GPH block, $3.50/gal
Total works out to $55/hour without fuel, $185/hour with fuel.
And compared to a single TP even then they end up a good deal. A PC12 will burn 70gal/hr doing 270kts. A MU-2 or Commander will do 54gal/hr doing almost 300kts. TBM will be faster, but it will also burn more fuel. An overhaul on a PT6 will never be less than $350K, even for the small ones. Probably closer to $400-500K for a single (P&W know how to charge for em as they have almost monopoly these days). You can do two Garretts for the same price.
So, if there's no cost penalty, you burn less gas, can go faster, can go further, have more useful load, why not have a twin?
You'll pay a little more at purchase compared to a 421C, but that's also value that you will get back when you sell, probably.