Any decision you make to learn anything based on payback has to factor in what will happen if more people choose to learn something. If more people learn Ruby because it pays better (let's assume that is correct) then supply and demand could equalize and the pay will go down. Right now if pay is high for Ruby it means most likely there is a supply and demand in balance. When that changes so will the pay. The only time I haven't seen this happen is with the rates lawyers charge. And the reason for that has to do more with personal relationships and quality of work.