How much do plane tickets go up if you don't buy them ahead of time?

I might need to fly from one major airport to another in the month of July. Problem is, I won’t know until 3 – 5 days beforehand whether I do or not so I don’t want to buy a ticket now. If I wait until 3 – 5 days, how much can I expect the prices to go up? (At about 160 – 180 right now.) Anyone have experience with this? Also, can you ever get refunds or transfers to the same flight but a couple weeks later?