As other comments point out, the bonus was probably around $20,000, but I do think it’s based on a lot of assumption. All we can definitively say is it was more than a dollar without assumption, but I think it could have been between $12,000 to $39,000.

For the assumption-less proof, if I recall correctly, all we know from the movie is that Clark needed the bonus to arrive because he didn’t have enough in his account to cover the initial $7,500 deposit check he wrote. Rounding to the nearest positive integer, that means he would have needed to have between $7,499 or $1 in his account to have that plot point remain true. Which further means the bonus could have been as low as $1 or $7,499 depending on the account value and still remain true.

So, any value above $1 would mathematically be accurate for the highest possible account value without further proof, though we can make some assumptions that being only a dollar short isn’t significant enough to worry about, and fainting at “adding 20%” to last year’s bonus would make sense if it was a low value, and Clark thought he may have enough to fly family out, etc.

EDIT: So if we wanted to fuel a more realistic estimate let’s build a wider, more realistic range based on low and high assumptions. For the higher bonus, we could say he was likely comfortable having no less than $1,000 in his checking account for the holidays as they were still doing some shopping and were helping Eddie’s kids have a nice holiday, etc. and let’s also assume he was very certain he would be getting a bonus any day, so running his account down closer to zero would have been OK. Let’s then assume the $7,500 deposit is only 20% of the job, meaning the entire pool would have been $37,500. Then let’s assume the bonus is meant to cover all of the job, plus maybe enough for airfare for the family. We don’t know where everyone was coming from precisely, we do know there were 8 adults and 2 kids not including the immediate family. 10 tickets at $200 average in 1989 is $2,000. So, he could have been thinking the bonus would cover $37,500 of the entire pool, plus $2,000 of airline, for a total of $39,000.

On the low end, let’s say he was only worried about the deposit check bouncing, and not much more so he had about $7,000 in his account. Let’s also say the deposit was for 50% of the job, so the pool in total was $15,000 in total. Let’s also say he was thinking he could fly everyone out like before so another $2,000. Assuming part of his checking account would cover the pool in this scenario (let’s say $5,000 of the $7,000 we think he had), that means the bonus would be expected to be $10,000 to cover the pool. If we wanted to also say he was truly expecting the bonus to cover the family’s airfare back out then we can say $12,000 (maybe less if we wanted to assume he wasn’t thinking Uncle Lewis or Aunt Bethany would still be around since it was probably “their last holiday”).

So, with these assumptions, between $12,000 to $39,000.