Scotts Cheap Flights has found some round-trip airfares from Houston to Europe in the $400 range. Travel up through May 2018, with some also in August 2018. Mostly Delta, some Air France and Air Canada.
European destinations from Houston include Brussels, Frankfurt, Geneva and Munich. Of course, you can take a European UCC (Ultra low-cost carrier, like Ryanair or Easy Jet) from any of those cities to wherever you want to go in Europe.
Other departure cities are also available, including Chicago (take Southwest or American non-stop from Austin to Chicago), Denver, Las Vegas and Washington, D.C.