It would be awfully naive to assume that credits will be as 'easy' to obtain in GT6 as they are in GT5 with the ability to purchase credits through microtransactions.
I say 'easy' regarding GT5 with irony because after nearly three years and hundreds of hours of game time I still have to 'grind' for an hour or two every few weeks(Bspec seasonal) to obtain the current car required for a series and I personally feel that is a bit absurd. Some may not, and that is ok.
This next part is pure speculation, but given the trend of most developers who offer cash microtransactions typically have available a $5, $10, and $20 option one could postulate the effects on GT6 utlising some costs that we know from GT5. GT6's announcement of a 500k, 1mil, and 7mil packages may be aligned with those $5, $10, and $20 figures, respectively.
Using those possible numbers above; let's look at the 'real' cost to compete in a high level GT500 series utlising figures we know from GT5, for example. The price of the car is ~$1mil. Now you break in that car and spend $500k on an engine rebuild. You then, presumably will require a set of tyres(or perhaps two if multi-compound) as well. You're now looking at over 1.5mil for this base car. Not done. If you want to be competitive in this fictitious series, you're going to need to test whether this car you built is faster with or without Chassis Maintenance(GT Auto), which is not removable. So you will need to build a second car(1mil) to test that back to back, this one with engine(500k) and chassis maintenance(500k). Oh, and tyres. You're now over $3.5mil. Still not done. The pointy end is also going to test the car with both the Chassis Rigidity(Tuning Shop) and the Chassis Maintenance(GT Auto) applied, and that is also non-removable again requiring yet another build(2mil, total invested 5.5mil). The 'typical' pointy end driver in this fictitious series may very well stop here at $5.5mil invested(not including the add on of 500k-1mil rebuilds after each enduro). The even more particular drivers will test this hypothetical car further, knowing that Chassis Rigidity(Tuning Shop) and Chassis Maintenance(GT Auto) produce different characteristics and will test the car with only Rigidity(Tuning Shop) applied and with only Maintenance(GT Auto) applied. There's another few million, if you'd like.
Before looking at the total numbers, somebody here will say that building anything past the base car is unnecessary. I am going to respectfully predecline that debate, because if you truely believe that the Chassis Maintenance and Chassis Rigidity do not appreciably affect the car's performance and driving characteristics; you're deceiving yourself. If you also believe that top level drivers don't test 2 cars(with and without Chassis Maintenance(GT Auto)) or more(if aftermarket wheels are an option) even for a Spec Race(tuning prohibited), you're deceiving yourself.
So, 5.5mil credits for a solid effort, plus 500k per race for post-race rebuild, and possibly 1mil for post race rebuild(if you determine that with Chassis Maintenance applied is faster). That's $20 real money for your first race(7mil cash pack) and $5-10 for each race you attend(depending on whether you require just and engine rebuild or couple with Chassis Maintenance). That could cost you $100 real money for an 8 race series. Hang around an autoslolam event and you can snag a day(or two) competing in a real car for that money. If you're the sensitive-to-changes particular-type driver who tests all of the combinations of options posted above, you're spending triple digits for 8 races on a video game. And I didn't even touch on the additional rebuilds after your testing sessions and fuel runs between races....because you are going to run a half-length race beforehand......right?
Keep in mind there are also popular series that run 3 cars per week and will require a minimum of one car be acquired per week(as it rotates out and is replaced) and sometimes two.
For those whom the above numbers don't apply, such as your car show/cruise types; see the following. Alfa Zagato @10mil credits in-game is going to cost you $28.55 in real money using the above hypothetical figures. Nearly $30 for one car. Even halving those prices is not acceptable, in my opinion. At that price point other driving or racing 'games' or 'simulators' become very attractive.
Of course, this study(used loosely) above may or may not be applicable, however it is not likely a far fetched conclusion. Again, even halving or further dividing these figures is rather unreasonable, to me. What some consider reasonable may be different than others, so please keep that in mind when reading this. Let's remember this is a video game.