well it depends on what "cost effective" means to you.
To me "cost effective" means the most reliable power for the buck. That would be the M20 built with boost. IMO. they are inexpensive to rebuilt and maintain. Even when rebuilt to stock specs they can handle significant boost levels with no problem. stock CR is 8.1:1 and I've seen M20's with 18-20 psi running on 91octane put out over 300 at the wheels, all while retaining some levels of relability and drivability. Then theres option 2: rebuild you M20 to a 3.0l stroker which has performance similar to an S50 then boost that.
It all depends what you want to spend. I could have rebuilt and boosted my old M20 for less than half of what it cost to do the Supercharged S50 swap, but I was going for originality and individuality. Not best bang for your buck.
All said and done a stock M20 rebuild should run somewhere around $2-$2500 and then a good FI setup would add another $2500 but you'd be over 300 crank hp for sure depending how hard you want to push it, 400hp IS possible.
As for an S50 swap, Bare minimum, with a high milaege engine would run around $5-5500. A low milaege engine swap would be closer to $7500. Then another $5000 for a supercharger and your up to about 360-370hp
A high milaege M50 swap can be done properly for about 3000.
But just remember after that $3000 you still only have mabey 200hp and over 150,000 kms, sometimes more. it will still need to rebuilt somewhere in the near future.
IMO. if you dont want to spend too much but still have some pony's to play with I'd go with the boosted M20. that way you basically got a brand new engine with no kms on it. and you wont have to worry about break downs.
Thats just my two cents,
good luck with your choices
costs are all in CAN currency