What’s up with R134 prices?

didnt know that.. can you add 1234 to 134 or have vacuum?
Your question is if you can add R-1234yc to a system with R-134A refrigerant? The answer is "no" since the EPA does not allow mixing refrigerants. All refrigerant must first be recovered by an approved shop tool. Oils must be changed if required. You then evacuate the system before adding the new refrigerant. Of course if all your refrigerant leaked out, there is nothing to recover.

Perhaps the EPA's most absurd rule is that if you change from R-12 to a HC refrigerant, you must first install R-134A, then evacuate it. They are also still "evaluating" the safety of HC refrigerant, going on 25 years now. But, you are allowed to buy and use HC refrigerant under federal rules, though some states or localities have their own rules outlawing it (laws pushed by AC shops who don't want an easy home fix). Whatever refrigerant you use, or will use in the future, I understand that PAO68 is the best oil since compatible with all, doesn't absorb moisture, and better stays put in the compressor for more efficiency (doesn't coat heat exchanger tubes). If working at home, keep looking over your shoulder for an EPA spy drone.