It's almost impossible to compare purchasing power from that long ago. Also, in 1702, the US was still a collection of separate colonies, and used British money -- but not at par. The value of the pound used in various US colonies varies, but was somewhere around 3/4 as valuable as a UK pound, so 20 pounds would be something like 30 Massachusetts pounds, for instance. Inflation since then has been something on the order of 20 or 30 times -- but Massachusetts doesn't use pounds anymore, of course!
Look at it a different way: how much did an average person earn in 1702? £20, or even less, was as much as a working class person might earn in an entire year. For instance, a footman might earn £6-8 per year! Of course, they'd have room and board included -- but it would be very rough living indeed -- a corner of a room to sleep, porridge or leftovers to eat. In contrast, a prominent London doctor would have earned £12,000 a year.
What do these people earn today? You might compare that footman's wage to minimum wage today -- working at McDonalds -- so maybe $10,000? And today, the richest earn much more than a thousand times that -- not doctors, maybe, but sports stars, singers, CEOs: $12,000,000 wouldn't be anywhere near the top. Some hedge fund managers earned nearly $1,000,000,000 a year until the recent financial meltdown.
See here for more info on wages in the 18th century in England:
http://books.google.com/books?id=o_hwrAoqxmQC&pg=PT1&lpg=PT1&dq=18th+century+salaries&source=bl&ots=LhOhv0aKHT&sig=F_XBVh-U8Wt4evjrazE3HxHguAo&hl=en&sa=X&oi=book_result&resnum=3&ct=result#PPA141,M1