Bilbo asked Dáin for as many coins as one strong pony could carry, and they were half gold and half silver by volume - one chest of coins of each type.
(Shrewd move, much better deal than an equal number of coins of each type, gold being 1.84 times denser than silver. Though it occurs to me now that if one chest was on each side of the pony, the load would have been very unbalanced).
Ponies can carry 20% of their own weight.
Pony weight range 130-350kg, so let's say a strong one weighs 300kg and could carry 60kg.
So Bilbo brought home 60kg of coins, divided 1.84/2.84 and 1/2.84 between gold and silver respectively.
That's 38.4kg of gold and 21.6kg of silver.
Now, our sole monetary standard in Middle-earth in the Third Age is that a clapped-out pony (again with the ponies) costs 4 silver pennies.
Tolkien had a lot of time for the Anglo-Saxons and was doubtless thinking of their silver pennies, weighing 1/240 of a troy pound (373g) = 1.55g.
So Bilbo's 21.6kg of silver was 13,935 silver pennies.
Let's say a clapped-out, but still driveable car costs £1000, so 1 silver penny ≈ £250, so the silver was worth £3.5 million.
For most of history gold was worth ≈ 10 times as much as silver.
The 38.4kg of gold would be 24,774 pennyweights (although Tolkien probably imagined it in the chunky 4g gold pieces current in the silver penny era, ie ≈ 9600 gold pieces, that doesn't affect our calculation) each worth £2500, total value £62 million.
So, Bilbo brought back to Bag End the equivalent of £65 million. Enough to buy quite a few fancy waistcoats.
