Well, when you say "bang for the buck" you have to consider engine life. The OS engines don't last as long as some of the others... 3-4 gallons under race conditions is what we've seen. So... for 2x the cost of the 3 port OS, I bought a Werks B7 Pro. Local guys are getting 8-9 gallons out of them, pushing them HARD... we're talking about sponsored pros like Hunter Kinsey and Mike Battaile.
In my experience, 7 port engines get better gas mileage... so that's something to consider too. My motor, with a JP1 pipe, and a 6mm restrictor, gave me 9:15 on a tank driving gently. Now, realize it wouldn't do that until I put about 2 gallons through it. They break in pretty slow. That engine is "softer" on the bottom than a 3-port, not as violent right off idle, VERY smooth power curve. When they spool up, look out! The RB engine makes as much power or maybe more (WS7 II/III) but it isn't as smooth throughout the power curve- the Werks has more USEABLE power.
Granted, those are higher-end engines... but they also last twice as long. My bud got 12 gallons out of a WS7 II.
Another of our local "Expert" racers is running the 7-port Go engines... he says 7 gallons or so, and they're only $199. That might be better "bang for the buck" than the Werks... and they're also VERY fast.
I'd stay away from the cheap 3 ports like the OS VG. Yes, you can buy them cheap, but they don't last as long. Novarossi engines last longer... but con rods can be an issue. Note that the RB engines are essentially modified, rebranded Novarossis. For CHEAP engines the Go engines look to be the current king, they seem to last a little longer.