This is never mentioned in the book but I'm wondering if there was a way to figure out how much gold Fred and George ended up winning, even if they didn't receive it.
I'm wondering if it got mentioned and I just missed it in the books?
Unfortunately we don't know. While we know how much Fred and George bet, we don't know what the odds were, so we can't calculate the winnings.
‘We’ll bet thirty-seven Galleons, fifteen Sickles, three Knuts,’ said Fred, as he and George quickly pooled all their money, ‘that Ireland win – but Viktor Krum gets the Snitch. Oh, and we’ll throw in a fake wand.’
‘You don’t want to go showing Mr Bagman rubbish like that –’ Percy hissed, but Bagman didn’t seem to think the wand was rubbish at all; on the contrary, his boyish face shone with excitement as he took it from Fred, and when the wand gave a loud squawk and turned into a rubber chicken, Bagman roared with laughter.
‘Excellent! I haven’t seen one that convincing in years! I’d pay five Galleons for that!’
Percy froze in an attitude of stunned disapproval.
‘Boys,’ said Mr Weasley under his breath, ‘I don’t want you betting... that’s all your savings... your mother –’
‘Don’t be a spoilsport, Arthur!’ boomed Ludo Bagman, rattling his pockets excitedly. ‘They’re old enough to know what they want! You reckon Ireland will win but Krum’ll get the Snitch? Not a chance, boys, not a chance... I’ll give you excellent odds on that one... we’ll add five Galleons for the funny wand, then, shall we...’
Mr Weasley looked on helplessly as Ludo Bagman whipped out a notebook and quill and began jotting down the twins’ names.
‘Cheers,’ said George, taking the slip of parchment Bagman handed him and tucking it away carefully.
Goblet of Fire - Chapter 7: Bagman and Crouch
That is how much Harry gave them (his Triwizard prize) to make up for them being cheated by Ludo Bagman.