While I can’t answer whether there are cheaper couplers out there, nor understand what you are talking about when you say
Originally Posted by
spectastic
…so you can only have 13,15 or 17 together. not 13 and 16...
This statement
Originally Posted by
spectastic
…If possible, I'd get freewheel on both sides, because fixed gear doesn't really interest me...
actually brings your project 1 step closer to being doable if you get your dropouts/trackends & chain right to work with the 3 tooth difference you are seeking.
I will leave commenting on how useful a 13t “sprocket” is in the real world, unless it is accompanied by a pretty small chainring to others, but the simple answer is a good old-fashioned BMX flip-flop hub. You put your small freewheel on the metric threaded side, & the bigger one on the side with English threading. IIRC, BMX hubs are 110mm? Just get one respaced to fit your bike.
Good luck…