Prepare bounty envelopes ahead of time, and separate them until you mix the correct combination of values together after the rebuy period ends. For the largest bounty, you could just label it "jackpot" if you do not want to produce a $350 and $400 envelope, and announce the jackpot value once the rebuy period ends. The following is a bounty schedule I came up with for a similar tournament previously, adjusted for scale (£20>$200). Give players a bounty chip for each elimination, but don't allow them to cash them in until at least the end of the rebuy period.
18: 9x $50, 6x $100, 2x $200, 1x $350
19: 9x $50, 7x $100, 2x $200, 1x $350
20: 9x $50, 8x $100, 2x $200, 1x $350
21: 11x $50, 6x $100, 3x $200, 1x $350
22: 11x $50, 7x $100, 3x $200, 1x $350
23: 11x $50, 8x $100, 3x $200, 1x $350
24: 12x $50, 8x $100, 3x $200, 1x $400
25: 12x $50, 9x $100, 3x $200, 1x $400
26: 12x $50, 10x $100, 3x $200, 1x $400
27: 14x $50, 8x $100, 4x $200, 1x $400
28: 14x $50, 9x $100, 4x $200, 1x $400
29: 14x $50, 10x $100, 4x $200, 1x $400
30: 15x $50, 10x $100, 4x $200, 1x $450
For more than 30 total entries:
For each 6 full entries greater than 30, add 3x $50, 2x $100, 1x $200, and increase the largest bounty by $50.
For the remainder (fewer than 6), do exactly ONE the following:
1 additional entry: add 1x $100
2 additional entries: add 2x $100
3 additional entries: add 2x $50 and 1x $200
4 additional entries: add 2x $50, 1x $100, and 1x $200
5 additional entries: add 2x $50, 2x $100, and 1x $200