$300, that's crazy.
6 years ago I spent $2k to get the bulk of possible mutations for CF tested, as my son came up having through standard testing as having one of the required 2 gene mutations, so a carrier. Being our first, there was the lingering question of whether we potentially both had a mutation and just got lucky that only 1 passed it on.
We didn't hesitate to go ahead with the testing. When the first (cheaper) round came back with common mutation on my wifes side, we figured it was still worth diving deeper to rule out the 1/100 chance. Knowledge gave us comfort in that instance, and the knowledge will give my son options in the future.
Learn what is useful, but I'd be cautious about going the full gamut. If it were me, I'd be concerned poking in the dark would just generate more worry than answers, unless there's something specific to look for. There are still so many unanswered questions on genetics.