Information: the discount in RMSS/FRP for skill DP for TP is 25% off, which is supposed to be balanced by a time requirement. Also if you take a TP more than once you only get 1/2 ranks but pay full cost DP cost.
That is a houserule.
I would not say it is a bad house rule, but still a house rule. I am actually using a version of it (no rule about half ranks if you take the TP twice) to avoid the problems with the TP costs. What is problematic with such house rule is that some training packages become so expensive that is questionable if they are realistic to purchase as all. As example you can consider the TP Doctor(V) that has includes skills worth of 54 DP. With a 25% discount you end with 41 in DP cost (compared to the books listed price of 23 DP or the 29 in the official TP cost list). The solution I use when using the flat 25% discount rule is that such expensive TPs is split into different parts so you can purchase it over a couple of levels instead.
As for the background of the DP mess. The real TP "cost formula" used in the first books is actually a mixture of many different discounts that was applied ad hoc by series editor John W. Curtis III. The reason the TP cost formula was not released was simply that there was no such formula in reality. This design became really problematic when companions was to be produced by independent authors and they could not get answers about the correct way to price TPs. Shiela Mguez and others on the RM mailing list did a massive job to come up with an actual formula that approximated how John W. Curtis III had done the costing.
The process they figured out looked a bit like this. The starting discount you get is between 25% and 0% depending on how much combat skills are included in the training package (a TP with five or more combat skills would get 0% in starting discount). Furthermore the cost is based on the cost of first rank in the category, the cost of second rank or the that fact that some professions should not be able to buy a second rank at all was not considered by John W. Curtis III. Finally there was cost adjustment done based on the total price of the TP if it was deemed to be too pricey. The ad hoc part was that John W. Curtis III did not always not apply the same final cost adjustment for every TP. The Doctor TP got an larger discount than many other TPs and it was not possible to figure out any pattern of when to apply a larger discount. Most probably the decision was based on how often the skill was used in his own roleplaying group. The actual discount applied in this step is as far as I knew between 0 and 60%, but I have not checked in detail.
What Shiela did was to determine the average final discount used in John W. Curtis III TPs and use that in a spreadsheet. This spreadsheet was approximately the same costs as the previous TPs and I believe it was used by the companion authors afterwards, but ICE never got around to update the core books so these continue to ship with the ad hoc costs from John W. Curtis III. In 2007 or Rasyr as series editor used the spreadsheet to calculate the cost of all TPs (that is TPS from books that by then was in print) and uploaded both the spreadsheet for TP creation and list of revised TP costs to the Vault.
Today your choices thus are:
1) Use the TP costs listed in the books that John W. Curtis III created with ad hoc discounts that are not consistent between professions
2) Use the official costs that exist in the Vault with consistent costs, but variable discount/benefit for each profession depending on the skill cost for the second rank.
3) Create some kind of house rule about flat discounts like I and many others have done to avoid the min/max playground that the above two choices present.
As a final comment...the time limitation to acquire TPs is problematic in practice since it only applyies during actual play. If one of your players are making a replacement character since his character died then there are no good reasons he can not say his character did have the time to take any number of TPs. Using TPs in this situation makes sense since you are creating a background for the new character, but all the discounts will mean this character become stronger than a character developed during actual play. It is a matter of opinion if this is a critical issue or not, but it is clear even with fair pricing of the TPs, the existence of discounts can be problematic in some situations.
