Are SSTO (Single Stage To Orbit) vehicles cost effective?

I've been experimenting with building rockets that can make it into orbit with a single stage, decouple and then land/splash the booster section with a probe core in control (or sometimes Jeb in a command chair).

They work rather well and I can get about half of the booster cost back but I find that my expendable asparagus booster designs are still cheaper to deliver the same payload to orbit.

Is there some payload amount or specific design that makes SSTO boosters more cost effective?

If you're keeping all of the booster parts intact, then the cost efficiency depends largely on how close you can land your booster to KSC. That's going to depend more on your skills at re-entry and landing than anything else.

There's another cost involved: the cost of the extra fuel you'll burn from getting a larger/heavier launch vehicle to orbit. However, fuel is pretty cheap; the cost to fill the largest tank in the stock game is about 6600 Kerbal Funds; that's only somewhat more than a single Mainsail engine.

If you're able to land your booster close to KSC and recover the majority of your parts costs, you'll likely be able to get better cost efficiency from an SSTO. Incidentally, this is why SSTO spaceplanes are worth doing; they make precise landings much more plausible.