Well I have to disagree with you on this subject.
You have to look at the average cost of the two trainers, if you kept reinvesting in a new one when it is cheaper to buy a new one. If you keep buying Brollos for $1.000 the best option is to buy a new Brollo when the old reaches a salary of 45398 or in week 8 (where week 0 is where you bought him), then you have to pay 1.000+45.398 (not 38.747)=46.398, the total salary for the 9 weeks is 38.747+39.522+.....+45.398=377.963, so the average cost of the trainer pr. week is
(46.398+377.963)/9=47.151.
With the other trainer it is more viable to keep him longer, in fact you should keep him until week 38, where his salary is 33.473 the total salary will be 918.518 for the 39 weeks. and the you have to pay the 433.473 when you buy a new one. Average cost pr. week: (918.518+433.473)/39=34666.
It can also be wise to get a "Yaseen" and keep him a long time, you can save a lot of money:-)