What is probably causing the progressively higher price is that you went into the settings and adjusted your max context up higher than it needs to be. As your conversation gets longer, it takes more and more context into account.

Most users just keep the context to 1024 in their settings. This prevents the cost from getting higher and higher.

Another way to solve this is to start new conversations each time you change your topic. This will reset the context tokens and will cost much less.

Does it make sense?