The least mehr which can be given is Ten dirhams(approximately 31 grams of silver). (Al Hidayaah).
Today the mehr value is calculated by using the weight of silver which would equal the amount of dirhams. For example the mehr fatimi is 131.24 tolas. (this weight is constant is will not change). But the value of this amount of Silver is continuously changing according to the stock market. The change takes place every month or so.
To be able to work out the approximate value for ten dirhams one can take the weight of the mehr Fatimi (131.25 tolas )and divide it by 400 (because 400 dirhams was the mehr which Hadhrat Fatimah was married with) and then multiply it by ten (to equal the weight of ten dirhams). This will give the value of silver which is equal to ten dirhams.
Then you have to find out how many pounds the silver is worth. This will change slightly as the value of sliver increases or decreases.
(131.25 ? 400 X 10 = the amount of towlas equal to 10 dirhams)
One tola = 0.375 troy ounces = 11.6638 grams.
One can use the method mentioned above to enable one to work out the mehr during different times of the year. The monetary value for 10 dirhams was approximately ?4.88 last time checked. This may have risen or fallen slightly since then, and is only given as a rough guide.