There is two very different things here : the value of currency, and the value of money. They AREN'T the same.
The value of a currency has NOTHING to do with the value of one unit of it.
That the pound is worth 1,5 $ or 0,01 $ has nothing to do with the real strenght of pound compared to dollar.
The value of a unit of any currency is TOTALLY ARBITRARY. It's decided by the government, which decide that now, this unit will be worth this number of this previous unit.
As an example : in France, in the 60s', the general de Gaulle (who was the president), decided the creation of the "nouveau franc' (new franc), which would be worth 100 of the current franc. It was a totally cosmetic and symbolic change, which affected absolutely nothing beyond removing two zeros from the prices.
The REAL value of currency is it's evolution. A strong currency increases its value compared to others. A weak on decreases. This is decided by the market fluctuation, which represent basically how much faith people have in a certain currency, and how much they think they can win by putting their money in it (depending of the interest rate).
A country with a strong and healthy economy (like Japan was some decades ago), means that its currency won't collapse soon. So it's a good and stable way to put money in reserve.
Another currency can come from a much less stable economy, but have higher interest rates. So it's less safe, but there is more to gain if all goes well. Increasing interest rates is a good way for government to make their currency stronger. But it costs a lot.
As the value of MONEY itself, it represent what you can actually BUY with your money.
If you gain 1000 $ a month, and you live in New York, you're going to be quite poor. Because just renting your flat will eat most of your income (let's say 700 $). Then the food is expensive, and the services in the city are expensive, then what you can get at Wall Mart is also expensive. You will probably have troube just to get what you need to live with so little money.
Now, if you live in Madagascar or Sierra Leone, you will probably pay no more than 100 or 150 $ for your housing. The food will be MUCH cheaper. So as services and goods. The bad side is that you will probably not be able to get the same goods as in NYC, but what you will get will be dirt cheap. With 1000 $, you will spend less than half on what you need to live, all the rest is bonus.
So in both case you get the same salary, but in one case you're a pennyless pauper barely able to feed yourself, in the other you have plenty of money left. So you can say that the "value of money" is higher in Sierra Leone, as you can buy more with the same amount.