I've been all over Mexico, Tijuana in the 80's where most of the gear came from including the famous inject ready horse needle Sust 250. Puerta Vallarta, Cabo, Cancun, Cozumel, Encinata and even Mexico city. Prices vary but I always compare it to what i pay for it. There is a lot of room for negotiation. Ive even walked out of the office where the pharmacist chase me down the street and said ok with the deal.
Also, there are many fakes out there an you need to be careful of that as well. As far as customs I've never had any issues.