The work takes place in 1797-1798, and Billy is a sailor on the high seas who is impressed onto the fictional HMS Bellipotent. At night, he is offered two guineas as a bribe by an unknown afterguardsman in order to advocate for impressed sailors, a bribe he denies.
How significant was this bribe? And how much would it be worth in today's terms?
A great resource for comparing historical prices in the UK, the US, and Australia is the academic site "Measuring Worth", which offers several ways to calculate the comparative value of currency. There are also some interesting essays there, for instance, on the cost of 19th-century slaves. It's a handy tool.
According to its calculator, 2 guineas (= 42 shillings) in 1797 would equal in 2014 (the current site year limit) £194.2 (US $279) in the real price of a commodity; £2,377 ($3,452) in the labor value of a commodity; and £3,275 ($4,756) in the income value of that commodity. I'll let the site unscramble those distinctions for you since such matters make my head hurt.