At what point does a player earn his prize money and points for getting to a certain stage in a tournament?

Your question contained several questions so I'll break them down and answer them one at a time.

Now if he pulls out before the match, I assume he doesn't get the money and the guy replacing him would get it.

That is correct - a "lucky loser" (someone that lost during qualifying rounds) would replace them in the main draw and that "lucky loser" player also would receive the rankings points and prize money for the result they achieve.

But what would if he limps onto the court and plays one point and retires, would he pocket 50k for doing that or would he need to play up to a certain point

It doesn't happen so much in smaller tournaments offering less prize money and rankings points, but the scenario you describe is common for the four major grand slam tournaments because of the money & points potential. If you were to look at the first round matches in any major that ended because of a player retiring, you will find some that didn't even reach the second set of the match. The player that retired (in most cases) knew - going into the match - that they had a health or injury issue that was going to prevent them from competing for an entire match. But the lure of an easy pay day usually has them play a few games before retiring rather than withdraw from the tournament and give up that money to another player. You probably wouldn't get the players themselves to admit to it - but that's what is going on.

How about if this is not the first round but after the tournament has progressed and therefore there is no one to replace him. Would he still get the prize money and points even if he retires? So say, the player wins the 3rd round then pulls out of the tournament, would he get the same prize money and points as someone losing in the 4th round?

It helps to think of points and prize money being awarded based on a player's wins, not their losses or matches played. So in the scenario you describe here yes, the player would get the same points and prize money for pulling out of the tournament after a 3rd round win as a player that lost a 4th round match. Both players won the same number of matches.