Yes you can purchase a one way ticket. There are several Low Cost Airlines in Europe such as Ryanair, Wizz, Germanwings, easyJet, Vueling, Norwegian - to name just a few. you don't say where you're starting from, but have a look at Skyscanner or Google flights and it will tell you which airlines fly that route.
Like many other countries, France does not require that you have proof of booked plane tickets. However, you must make sure to let the immigration officers know that you will be leaving the country and do not intend to stay for an identified period.
What is this? So while you can technically travel on a one-way ticket, they also need some kind of official return ticket confirmation showing that you are leaving the country eventually. They won't necessarily care where that ticket goes, just as long as it's out of their country.
Requirements for British Citizens Travelling to the EUValid passport. Health insurance (such as private travel insurance or an EHIC/GHIC). Enough money to cover your stay in the EU. Return or onward travel ticket.
You must arrive in country with proof of sufficient funds and a return airline ticket or other evidence that you will depart Greece if asked. The sufficient funds requirement is often met with credit cards. U.S. official and diplomatic passport holders must obtain a Schengen visa prior to arrival.
In other words, if you visit Spain for a short period of time without having a residence permit, whether your country of origin required you to apply for a tourist visa or not, you can stay for a maximum of 90 days before you actually have to leave or obtain a legal residence permit.
If you travel without a valid ticket you have to pay a 60-euros-penalty fee. If you feel treated unfairly, you should immediately raise an objection with the transport company.