In my current campaign as the UP, I purchased New York from Britain for 70k gold plus the province of Gujarat in India. That's the cheapest monetary price I've ever paid. I got Bengal for 75k plus a couple of techs, New Andalusia cost me 142k, French Guyana cost me 117k, and Carnatica cost me about 80k plus techs. I don't recall precisely how much I paid for Goa, Flanders, or whatever the name of that province is that lies between Carnatica and Bengal (I think that one was around 160k because that's what I had at the time).

Interestingly, in the cases of New Andalusia and French Guyana, the seller came back with a counter offer to my initial offer, both times mysteriously just less than the total amount of money I had at the time. It's almost like the AI knows exactly how much money you have in the treasury.

The AI constantly tries to trade 1-2 techs plus a few hundred gold to me for territory, but it won't let me purchase one for less than an exhorbitant price. Since I'm making about 45k per turn at this point in profit, I can buy a province about every 2-2.5 years, and there's no "Territorial expansion" penalty to diplomatic standings when you do. I am just about resigned to having to conquer India to win this campaign, though. No matter how much money I give the Maratha or the Mughals, they won't go out and fight each other, so I'm not going to be able to buy one of the capitals from the victor. I think I'm going to have to go take them myself.