My vet recommended using poly g for my horse instead of adequan. He thinks because of his age it will work better. Has anyone here used this?

Yep! All the time! Works great and cost effective. Mine get it monthly and before Rodeos. There are a few threads on it on here if you search. 

He advised he would just give it to him 48 hours before a race...but the vet sells it at $70 a dose so that would be $140-$210 a month. Do you only do once a month or do you do a loading dose?

No loading dose. It lasts longer than legend etc. I give monthly and I buy it online for 90 a vial, 2 doses. If I have an extra hard weekend or multiple runs etc, I will give another dose mid week of the rodeo week.