Offer 20.00 Us and be firm you will get one of them to take you .They will take us currency so dont worry about that. Once you are in Cabarate there are 2 banks you can use to change money, both on the main road. I usually use my debit card at the bank atm instead of changing dollars, but my bank only charges 1% for the fee yours may be different. If you spend us dollars with the local merchants they may give you a lower exchange rate, they make a few extra bucks this way so try to use only peso when shopping.
Always ask for a cash discount some will give it some will not but it is worth the ask.