Not all II ACs are created equally. And remember that II also sells Getaways.
Yes, sometimes they will offer an AC for depositing a desirable week. For example, if I lock off a Marriott 2BR into its separate 1BR and studio, I may get 2 ACs from that deposit if Interval has a promo for deposit running. In general, these types of ACs seem to have more "power" to pull better weeks, and also may have a longer term until they expire. Also, these may allow you to place a search request.
Sometimes II unexpectantly drops 1 or more ACs into your account, as mentioned above. These are of variable duration. It's not always easy to find a desirable trade because II is offering the less desirable weeks for these ACs.
The cost on an AC trade varies, usually from about $250-ish to $400-ish. Some of our past AC trades have been gems, but we've had some duds.
I have 2 II accounts, so I have a bunch of ACs at the moment. I don't know a quick way to automate searching, so it can be time consuming to search with multiple ACs. ACs tend to have different fees, so if I do find a week I might want, I then check if it's available via my different ACs and select the least costly.
Also, you can't always assume that an AC is the cheapest or best way to go. Occasionally I have been able to purchase a Getaway for less than an AC trade.
We've owned our timeshare since 2004. I have learned a lot about the timeshare game since then (thank you TUGgers), but I still find the inner workings of Interval International somewhat mysterious.