Also if you do buy do it in groups of 3, which is 10% off, 6 is 20% off. Find one of the promo codes which you like. The MX-5 Cup car is a good way to start as jjaisli mentioned.
I am not sure how many of these codes still work but see which ones you like. You can only use one.
3 months basic membership for $9 : PR-3for9
Regular retail price : $30
Discount:$21 (70%)
12 months basic membership for $49: PRLEAGUE12
Regular retail price: $99
Discount: $50 (50 %)
3 months basic membership with the NASCAR Cup Car, Daytona and Talladega for $35 : PR-CarbCup3
Regular retail price: $71,85
Discount: $36,85 (50 %)
1 year basic membership with the NASCAR Cup Car, Daytona and Talladega for $75 : PR-CarbCup12
Regular retail price: $140,85
Discount: $65,85 (45 %)
3 months basic membership with the Williams FW31, Silverstone, and Zandvoort. for $35 : PR-GPWire
Regular retail price: $71,85
Discount: $36,85 (50 %)
One year basic membership with the Williams FW31, Silverstone, and Zandvoort. - $75

R-GP
Regular retail price: $140,85
Discount: $65,85 (45 %)
3 months basic membership for 12 $US : PR-2FREE or PR-GA12
Regular retail price: $30
Discount: $18 (60 %)
1 month basic membership + the HPD ARX 01c and Spa for 15 $US: PR-INTELGT
Regular retail price: $38
Discount: $23 (60 %)
1 month basic membership + Zandvoort for 5 $US : PR-Zandvoort
Regular retail price: $26.95
Discount: $21.95 (82 %)
3 months basic membership + Dallara Indycar for 24 $ US : PR-Indy
Regular retail price: $41.95
Discount: $17.95 (43 %)
1 year basic membership for 69 $US : PR-SAVE
Regular retail price: $99
Discount: $30 (30 %)