If you like them all fully upgraded in your garages and you finish both series ( exhibition and Season ) you will have to invest: 5052 ( upgrades ) - 400 ( LTSses ) - 75 ( Season ) - 10 ( Exhibition ) = 4567 GC.
Cheapest way to get them all 4: 4 x 328 = 1312. And you can get the 10 from the Exhibition and a bit of the season series.
And you can do it even cheaper. Only go for the Acura with a PR sufficient to finish the 2018 Season. ( For those with not so much GC ): 853 GC ( upgrades ) - 100 ( LTS reward ) - 75 ( Season reward ) - 10 ( Exhibition ) = 668 GC.
Or only go for one ( the Acura ). Only 428 - 100 - 10 - ?? ( Don't know how far the LTS PR will bring you in the series ). So max. 318, but probably under 300 GC.