In reality, Christians of a certain ilk believe the rewards they can reap in heaven dwarf any conceivable monetary amount.

They will certainly say they do. However, they don't generally behave as if they really truly deep down believe that. In particular, I can think of very few occasions when I have seen Christians acting more motivated by the prospect of heavenly rewards than they would have been by an offer of $1M in a year's time.

Note also that a typical Christian facing an injured biker probably doesn't believe that he will be saved if he helps and damned if he doesn't, Matthew 25:31ff notwithstanding. Quite right, too; after all, he will be faced with plenty of other opportunities to help or not help, and it can't possibly be true that each of them determines whether he's saved or damned.

And note also-also that this isn't really about Christians versus everyone else; it's about people who believe in huge post-mortem rewards and punishments versus everyone else. E.g., Muslims' beliefs on this score are broadly similar to Christians'.

It doesn't follow that humans don't ever act based on incentives.

And did anyone claim that?

The amount is arbitrary.

Yes, and in the case of both A and B helping the bloody biker, the amount doesn't matter, even if it's arbitrary.

In other words, if you place two spherical cows in vastly different scenarios with remarkably different variables

First, no, they are not spherical cows. Bloody people actually do stand by the side of the road from time to time. Strangers stop and help them. It's an empirical phenomenon which you can empirically investigate.

Second, the "different scenarios" is the entire point. You asserted that Christians will be more likely to help than non-Christians with the implication that this distinction is important. Before that, Alia1d said she'd expect less help in a non-Christian country meaning there would be an observable difference in outcomes.

I think both these positions are wrong. Being a Christian or a heathen is NOT what determines the percentage of random strangers who turn out to be kind-hearted. Other factors, like the ones mentioned by me, are much more important. And similarly, whether you get help on the side of the road or not in a particular country is not driven by whether the population is Christian.