Sam's Racehorse - the answer
The answer is really not so hard to figure out
The asking price for the horse was twenty guineas, they settled on eight guineas, but three of those were dud, so he only really paid five guineas. Five guineas is £5/5/-, so he had enough from his £7/14/2 Army retirement pay to buy the horse.
He sold the horse for 40 raffle tickets at 5/- each, which means he collected £10. He gave 5/- back to the winner, so he then have £9/15/-. Subtracting the £5/5/- he paid for the nag, his profit was £4/10/-.
We know from the previous page that £4/10/- in the current denominations is £4.50p, but how much is that worth today?
Guinea coins were withdrawn from circulation in Britain in 1816 and we know from previous poems that Sam was in the British Army fighting in the Peninsular War of 1807-1814. From that we can deduce that the transaction took place around 1815. Using Britain's historical retail price index, £4/10/- in 1815 was worth £265.86 in 2008; enough to buy a decent bicycle.
And here's another puzzle for you:
Q: how many times can you subtract 7 from 83, and what is left afterwards?
A: I can subtract it as many times as I want, and it leaves 76 every time.