The price for Forza 6 is $59.99
The Price for GT6 is $14.99
The difference is $45 dollars at this moment, so you are quite literally right saying they have a price difference. You're not right in the way you approached the situation though, as something that is completely optional should not be considered when talking about price of two base games.
The arguement of DLC is not a great one, as it's optional and is not required in any way whatsoever to enjoy the vast amount of cars that the base game already has. If you don't feel like paying for DLC than dont as you're more then likely going to get them in the next iteration(or some kind of different trim of said vehicle, at least.) The amount of free DLC in the past Forza games have already rivaled that of PD. This trend has continued for iterations, so I don't see it stopping here.
The season pass covers 6 months worth of DLC, which is around $30 if I remember correctly.
Fact: Paid DLC is optional. It was in GT5, it is in FM6.
Fact: The Paid DLC so far in FM6 totals $30 - if you've bought the season pass (42 cars). If you're buying each individual pack, the three so far total $21. There are still three more packs to go to cover the season pass, and as many as nine more total.
Fact: FM6 has also had free monthly cars (Infiniti Q60, NISMO GT-R LM, Silvia).
Then If you compare both games taking only their buying price and nothing else, then exclude all the DLC's and the season pass and everything that its not free.
In GT6 absolutely everything has been free so far and you only had to buy the game... including online of course. So give credit where its due.
GT6 clearly more value for the money than forza IMO