You didn't mention how long they are planning on being there. If it is only a month, no problem. I had my daughter meet me this year in Spain and she only had 7 days vacation. She flew in and out of Madrid. We went to San Sebastian, Cordoba, Toledo and Seville and back to Madrid. It was a whirlwind tour. I just mention this because depending on the length of their visit will really narrow down what they can see. It was almost too much in 7 days, she wished she had more time in a few cities.
Another thing that will affect the cost is what city you fly into. LHR (london heathrow) for example is a really expensive airport to fly into. Their airport taxes are a lot higher than, say Madrid. Flying out of LAX will give you the best choice of flights to Europe. You can't see everything in one trip. You can only stay in a Schengen country for a TOTAL of 90 days, then you have to leave for 180 days. England is not part of Schengen so you could spend 90 days in Spain then go to England and spend another 90 days before you would have to leave. There are MANY posts on this on Tripadvisor if you want more info.
I found Spain and Portugal very affordable, Portugal more than Spain right now. It will probably be different come June 2016 due to the situation in Europe.
If they are going to go to Eastern Europe, check into the Visa requirements well ahead of time. Get a Passport early if they don't already have one. Get any necessary vaccinations ahead of time - find out what you need, it depends on where they go. Tetanus booster for sure, and maybe Hepatitis B&C - those require 2 weeks between shots and a series of 3. ( I think that is Hep b) Probably not much else. If you belong to Kaiser, they have a travel nurse you can talk with that will tell you if you need any vaccines by discussing the trip with her.
I travel 365 days a year, and have gone around the world twice now. Feel free to email me by PM if you want to ask me a question.