How much was 50 pounds worth in 1749?

It was worth 50 pounds.

English currency was a 'world standard' currency at that time; that means it did not really change its value. At that 'time' gold was worth 33.33 per troy ounce, so you could (arguably) convert current gold prices to 'pounds' of buying power on that date; to get a general idea of what it was worth.