In terms of sterling and US dollars, there are two main methods most historians would use for converting prices from historic values to ones modern readers can understand: by adjusting for consumer inflation, or by measuring the value of a sum relative to gross domestic product.

When we compare by consumer inflation, we measure the value of money in terms of what it can buy - depending on the time period, we can determine this either by tracking the cost of a bundle of goods the average household buys, or by the average price of goods in the wider economy. Numerous historical economic studies have estimated the value of key goods on a year to year basis, and we take these as a rough indicator of how the value of money has changed. This is made easier by the fact that inflation as we understand it today is a relatively recent phenomenon; whilst price variations take place from year to year, long-term inflation is a very modern thing indeed. $100 in 1790 would get you the same as $100 in 1890; in contrast, $100 in 1890 is the equivalent to at least $1,480 in 1990! This is largely a consequence of having a currency pegged to precious metal; the silver and gold standards placed significant constraints on the growth of the money supply, which tends to result in long-term price stability, with periods of both inflation and deflation.

Consumer inflation only tells us what money can buy though - it doesn't tell us what money is worth. To gauge that, we look at gross domestic product (GDP) - the approximate value of everything a nation produces in one year. GDP essentially tells us how much money is going around - it accounts for consumer spending, investment, government spending and the balance of trade (exports minus imports). Just like with the price of goods, we have numerous studies estimating the GDP of the USA and the UK as well. When we consider the value of something, we can also consider what share of GDP it was. For example, as I said above, in terms of what it will buy you, $100 in 1790 is the same as $100 in 1890. However, by 1890, the US economy was significantly larger and wealthier - there was more money around. As such, someone who had $100 in 1790 is actually about as wealthy as someone who had $8,000 in 1890, because in an economy with much less to go around, actually being able to get $100 is much harder - even though it would buy you about the same amount of stuff in both years!

The first method is more useful for helping people to understand how much things cost; the second is more useful for providing the context of worth. The problem with both methods is that they are ultimately estimates, and quite rough ones at that - even today we cannot perfectly measure GDP (if anything we probably underestimate the size of our economies today, because GDP measurements in most nations do not account for all manner of illicit economic activity). Furthermore, when working as historians, we generally don't try to convert prices - where we present figures in modern-terms, it is to try and help laymen to get their heads around the significance of the values we're talking about.

There are other measures we can use also in other contexts - like estimating prices based on how much work the average unskilled worker would have to do to buy something. This website is a fantastic resource maintained by a panel of historians and economists dedicated to converting monetary values in Britain and the USA, as well as a raft of other indicators, which you might find interesting.

A former colleague of mine would open her intro European history class by doing calculations on how much Columbus' voyage to the new world "cost."
First, she would run through the costs in gold (for 3 ships, crew for them, naval stores) and adjust gold prices for inflation; = high

then in silver = very high (...because this was before the flood of silver from south america, obviously)

then in shipping costs (using Genoese prices for the mediterranean trade); = medium

then in labor cost (for building the ships and crewing them) = very low

then in raw materials cost (wood for the ships, bread and water for the men) = very, very low

It was very interesting, especially as it was an indirect way to show how VERY difficult it is to compare prices, costs, and/or standard-of-living across centuries. Wish I'd copied down the numbers.

Early Modernist here.

I haven't exactly converted seventeenth century prices to modern ones, but I have tried to place contemporary prices in a context so that we can have some understanding of what something is actually worth.

The usual approach that I take is to look at the prices of basic essentials, such as a loaf of bread. There will also generally be some evidence of what, say, a skilled artisan or a common soldier would be paid on a daily or yearly basis.

Based on this type of data you can then make comparisons and have a reasonably good idea how expensive something was by contemporary standards. Of course, you also have to make allowances for things like general costs of living; how much disposable income would a person have? In addition, especially amongst the lower classes, the economy would often still be less based on actual money, and more on an exchange of labour/skills/services.

I don't think there's much point in converting such costs to modern day equivalents, as I think the context of the price of something is far more important.

I know you specified "not the recent stuff", but something I've found interesting is that in the UK, post WWII, You can extrapolate relative prices using the cost of a Mars Bar! http://www.ft.com/cms/s/2/9f128868-68b4-11da-bd30-0000779e2340.html#axzz3W9xkS7XO