What is the potential cost of fixing a coolant leak within the engine?

If you are getting coolant in your oil, stop driving it, as you will be sure to cause damage. If you can't afford or don't want to invest the money in replacing the head gasket, there are treatments, though they may not work or work the best, that you can use. Many auto parts stores will sell block repair or head gasket repair items, some liquid and some that are like ground flakes. These work by gradually plugging the leaks through the gasket and they harden while in the hotter temperature of the engine, so they are not suppose to harden in your cooling system, though there is always the chance of causing buildup when putting those in your system. I have known of some people that swear by these, that they work without fail, but something you will have to decide on. But be sure to do something as coolant in your oil means your oil is not working as it should, and that can lead to much bigger problems. I included a link to one such item, though local stores will have a variety.