It's usually cheaper to buy a roundtrip ticket to France than a one-way ticket from the USA, at least. Perverted, but true. I just checked from one east coast US city for March 29 departures. The cheapest roundtrip ticket to Paris was $388. The cheapest one-way ticket was $860.
I've never been asked for proof of an onward ticket anywhere I've flown, including France, and I've flown to France quite a few times. If you have the good fortune to hold a passport from a first-world country, you'd probably have to really look like a complete mess to be questioned at all.