Not a flaw in his logic, just a different criteron for evaluating the economic benefit of his purchase than what you're using. Further, I'd say his method is more "correct" than yours in that it's a clear, concise way to determine the value of the new staff member. His method could even be extended to handle any length of acceptable payback period via simple algebra. Assuming you replace with the same level (specialty doesn't really matter, but assume it's the same)
Let SC = this week's salary of the current staff member
Let SN = the listed salary of the new staff member (which will be next week's salary -- there is no increase on a staff member's first week with you)
Let L = the level of the staff member
Let W = the number of weeks since the new staff member was hired (0 will be the week you hire him)
Let X = the number of weeks you want your payback period to be
Then your weekly savings for the new staff member for the first week is:
SC*(1.0075+L*0.0025) - SN
And your weekly savings for the new staff member for the second week (W = 1) is:
SC*(1.0075+L*0.0025)^2 - SN*(1.0075+L*0.0025), or (1.0075+L*0.0025)^W * [SC*(1.0075+L*0.0025) - SN]
So, the total savings of the new staff member can be summarized as the sum, for W=0 to X, of (1.0075+L*0.0025)^W*[SC*(1.0075+L*0.0025)-SN].
With respect to this sum, [SC*(1.01+L*.0025) - SN] is a constant, so we can rewrite the sum as:
[SC*(1.0075+L*0.0025) - SN] * the sum for W=0 to X of (1.01+L*.0025)^W.
Now, since the factor (1.01+L*.0025) is greater than 1, the sum is going to explode without bound (albeit it fairly slowly in BuzzerBeater-time). But, since that piece is greater than 1, the total amount saved for any number of weeks X will be greater than:
X*[SC*(1.0075+L*0.0025) - SN]
Acquisition costs are the price paid (PP), and the current staff member's severance (SC). So, as long as the following holds:
X*[SC*(1.0075+L*0.0025) - SN] > PP + SC
then your new staff member will cause you to at least break-even over your desired financial time horizon.
Or, put more simply: You paid a rational price for your new staff member as long as the price paid plus the termination charge for your current staff member is less than the weekly savings for the new staff member multiplied by the number of weeks you desire for getting paid back on your money. So, in my case, if I have a 40k L5 trainer and I want to replace him with an 18k L5 trainer, and have a pay back of 1 season (14 weeks), my maximum "rational" price to pay would be 268k (22k * 14 - 40k). And in reality, it would be slightly higher than that, due to the increasing payback of the lower salary trainer (the 18k's salary will grow slower in absolute dollar terms than my current 40k salary -- over a 14 week payback, the real sum of the (1.0075+L*0.0025)^W series is about 16.3 for a level 5 trainer, which would put my fair value at just under 320k)