The 50% isn't capped at 50k, however arresting somebody worth 150k will net you a 50k bonus.
If 150% of the bounty is worth over 200k your bonus is equal to 200k minus the value. So really you want a bounty worth as little as possible while going over 200k combined with the bonus.
Say you arrested somebody worth 135000, the bonus would be 67500. 200k - 135000 = 65000. So you get a bonus of 65k, whereas if you arrested somebody worth 150k you would get 50k back. (200k - 150k)
Long story short, the most you can get from a single crusade is 200k (Including bounty+bonus). If you are keeping the crusade bonus to yourself but splitting the arrest with peers it is more profitable to hunt bounties >133500 but <150000
There also is no larger bounty then 150k, so assuming you are keeping the crusade bonus to yourself it is always more profitable to get the bonus over the 15%