The Planetary Society has an interview with Ed Weiler, associate administrator for NASA's science program. He discusses the thinking behind various Mars mission opportunities. Two interesting quotes:
"Weiler pointed out that "2018 is an excellent launch opportunity due to a very favorable orbital alignment", much like 2003 was when NASA launched the twin rovers Spirit & Opportunity which function still to this day 5 years later. He envisions a "very good possibility to send two rovers/orbiters by ESA and NASA, one of which could be equipped with a methane spectrometer". "
"The trade off to accomplish dispatching new spacecraft in the 2016, 2018 and 2020 timeframe would be to basically "take Mars Sample Return (MSR) 'off the table'. We'd have to give up on MSR for now", Weiler said. There is simply not enough money in anyone's budget to realize all these objectives simultaneously. "The cost of MSR would be on the order of at least $6 to 8 Billion and with a timeframe of the early 2020s". There is a debate within the Mars community on the wisdom of building one big Flagship class mission in a decade vs. multiple smaller and more focused missions more frequently."
Read the full interview at: http://www.planetary.org/blog/article/00001940/