In the end, the three men have only paid £27, not £30. The £2 kept by the bell boy should be subtracted from the £27, not added to it.

Try it with different values:-

The room now originally costs £33, so each man pays £11. The cost, once again, drops to £25, so the total refund due is £8. The bell boy keeps £2 and refunds £6, thus each man has now paid £9 again, for a sum of £27, and the bell boy has still got his £2, giving us our total of £29 again, only this time we're £4 adrift instead of £1.