You say that like it's a bad thing.
Only if this massive an expenditure results in the usual game which seems to be in perpetual beta-test, like the previous three games were, and GT5 is.
I can see why both games have such big budgets. GT5 has a lot of technology running at once which one of them, 3D, really should have been an OPTION to install, as very few of us have 3DTVs yet. And if you look at the graphics of Prologue again, you'll see that it's better looking than GT5 in a few ways. Plus the insane level of modeling and art involved in the Premium cars and tracks are only being matched by Forza 4 now.
Forza has expansive backgrounds which have more in them than GT5's, with some sort of well done LOD system which provides just enough detail to make it look realistic, at least from the YouTube vids I've seen. This might be something the war shooters have developed, and something PD could pick up on for GT6, but if it's something T10 themselves worked on, then good on them. But nothing Dan G or their demos ever previewed prepared me for the frooky things buried in the games, and I'll be holding my virtual breath as I get into F4 for the little beta-bombs usually in every MS software, including the last three Forzas. Here's hoping they're squashed this time before release, or there are going to be the usual leaderboard wipes and other fixes along the way.
And on those other racing games, CSLACR is really enjoying throwing a lot of disparate info on the whole "what racing games are like what" banter by intentionally throwing all kinds of random bits together and blurring distinctions. Of course he's got me on ignore now, so I don't have to worry about hurting his butt with this.
NFS is based on Cannonball Run and Gumball Rally (movie) racing in which hot rods and supercars race over various terrain which no serious racer would. And with the limits of PCs - and developers - even today, EA couldn't make extremely long point-to-point courses, so the game took place in circuits instead, though in outlandish scenic locations from city streets winding through warehouse districts and aquariums to mountains and canyons. And we all remember the Hot Pursuit games with lovely cop chases, which some people can't seem to get enough of. This style of racing in the gene pool runs in a lineage through games like Midnight Club, Tokyo Extreme and Project Gotham, and can even be argued Burnout and Motorstorm.
Gran Turismo was based loosely on the concept of the entire bloodline of real world motorsports, from enthusiast motocross clubs with their stock weekend racers to the world of professional racing, without the cop chases and fireworks of the arcadey NFS and similar games. It used real cars and realistic race track settings in contrast to the flamboyant content of the previous arcade racers. It was so refreshing from the usual racer, either arcade NFS style or stuffy PC sims with their restricted car lists and options, using real world sports cars, and gave the player a range of realistic upgrade options to tune their rides and make them competitive with more powerful, higher performance stock cars, and even race cars to an extent.
Forza had a bit of this NFS flavor in the first game, using a street circuit from one of the NFS games, "Fujimi" (Mt Fuji) for a mountain point-to-point series and even Junkie XL for the soundtrack, a long time provider in NFS games, and is most likely because of the members hired away from EA's NFS team by Microsoft to found the core of Turn 10 as mentioned years ago in the Forzacentral discussions with me by one of the mods there. But the rest of the game, and very distinctly so in the next two Forzas, followed the Gran Turismo formula closely.