The Victorian Web, based on careful analysis, recommends estimating value by multiplying costs by 67x in 1880 and 72x in 1890; perhaps 70x would be about right for 1888 (as they note, rounding to even 10s makes little difference anyway). Obviously this gives a price in modern pounds, which you must then convert again. The point is that if something cost £2 10s., that's about £175 in 2002 when they made their estimate, and thus let's say $340.