One thing that Turn 10 and Microsoft snuck into Forza Motorsport 5 was microtransactions, whereby you can spend money on Tokens and then use them to acquire cars in the game (you can also use them to increase the amount of XP you receive for each race for a limited period of time) if you wanted to shortcut the career to snag the faster cars quicker.
For 2,334 Tokens you can buy either a McLaren P1 or Ferrari Enzo. A Token pack costing £15.99 will cover either of those cars (you can buy packs of Tokens at the following prices: 100 is £0.79, 575 is £3.99, 1250 is £7.99, 2700 is £15.99, 8000 is £39.99 and 20,000 is £64.99), so with the 2700 Token pack that's one vehicle for £13.82. But there's much more expensive cars in Forza Motorsport 5, but the more Tokens you buy, the cheaper they are. If you bought the 20,000 pack (costing £64.99) the McLaren would cost just £7.58.
From the same pricing, a Veyron Super Sport will set you back £13.00, and the most expensive car in the game, the Lotus E21 will cost an incredible £32.50. The smaller the pack of Tokens, the more expensive they are, so the worst possible way of picking up the game's most expensive car would be to buy 100 of the 100 Token packs, bagging you the exclusive 10,000 Token Lotus E21 for a cool £79.