Ichiro has 25 games left, and he needs 32 more to set the record.
This year has been exceptional for him - he has 224 hits in 135 games. If you expect him to continue to hit that well, he has 41.5 "expected hits" in the rest of the season, if he doesn't get injured and isn't rested for a game. If you accept these assumptions, your prop has a 90% chance of winning using a 1-variable poisson distribution (mean of 41.5 vs. 32 needed).
Batting averages fluctuate wildly from year to year. In the last 3 years before this season, Ichiro has batted only .328 (doing worse with time). If Ichiro hits only .328 for the rest of the year, and he misses one game, that lowers his expected number of hits to 34.5., and your prop has about a 68% chance of winning.
Either way, -201 has value, and yes, one game (with 2 hits) can dramatically change the line. Of course, it can move violently the other way if he has a no-hitter game.