That's a good question.
Cheapest that's plug and play:
Motamec Formula D Shaped Wheel ($70 USD) + PenguinRC SimuSERIES GT with USB box and the cheapest shifters $279 = $350 shipped.
Cheapest that you can just build yourself:
Motamec Formula D Shaped Wheel ($70 USD) + DIY 3d printed button box (search Thingiverse, but there's some higher quality ones by Turn Racing), maybe $100 with materials/electronics. For the USB board you can use a Leo Bodnar BBI-32 if you don't feel comfortable wiring up a PCB yourself. If you want nicer shifters, you can use the Ascher Racing bolt pattern and pick up Ascher magnetic shifters.
Cost effective but medium/higher than the others quality and components:
SRC GT1 (Probably around $400 shipped) by Sim Racing Coach. Includes a GT style D shaped wheel. compared to the PenguinRC button boxes, it has rotary encoders and knitter switches (more tactile click buttons).
Be aware that if you get a 3D printer, you can build some pretty good quality button boxes and choose how high quality you want the components to get. They have Thingiverse files for Porsche 911 GT3 replica wheel plates, other more universal versions. You can even 3d print your own magnetic paddle shifters. I have some on my custom button plate and they're quite decent for the cost.
My recommendation if you want it easy and not expensive - SRC GT1.