It is the way it works. My son's summer team shuts down in fall and I don't want him playing the full fall schedule, so we usually end up doing it like this. He usually just comes in, starts a game and comes home. Prices can range from $0 to $450.
Whether it is a good opportunity depends a lot on what organization is making the offer. You want him on a team with a roster full of studs so he gets looks from schools there to see other talent (not to mention mlb scouts if he's at that place in his development). $300 is in the upper end of costs, but not completely unreasonable.