I know that programming an AI is hard, but doing it for monopoly should be relatively easy. It is a game with a clear optimal strategy and it only gives you a handful of things that can be called real choices (it gives you plenty of false choices, but the AI should easily be able to know about those).
At the start of the game, do you skip buying low income properties? If so, which ones? How about later in the came?
When is it ok to buy houses? How many house should be purchased and how much money should be kept in reserve? When is it ok to mortgage a property or sell a house to buy another property?
When is it ok make a trade for a higher income property that involves increasing the chance that they will gain a monopoly, and vice versa? How much cash is it worth to buy/sell something that gives you/them a monopoly? What about trades that involved multiple properties? Is it ok to give them two lower income monopolies in exchange for a higher income one?
The base strategy is "always buy what you land on". Exception being when two players own property of that colour, then it can be debatable.
When trading, don't trade high-valued property if that means that your opponent can get all 3 of a colour unless you can fill multiple properties of your own, or you are given a lot of money for it.
Until most property has been purchased, pay yourself out of jail. Later on, when properties are well developed, never do it.
If opponenet has more high valued properties than you, cause house shortage.
Don't buy houses if it is likely that you will end up loosing too much money in the next couple of rounds. If it is unlikely that you will land on lots of "bad squares", buy houses.
If you get this down, then it is more down to luck than anything else. Sure, there is some fine-tuning in how much you should trade for for optimal effect, but Monopoly is a game that looks like it is more strategic than it really is.