I would guess that the money was probably inherited rather than earned -- likely from James, as Lily was muggleborn. Even though they may have been a power couple, or as you say, a highly skilled witch and wizard team, they were only 20 when they died -- not really enough time to amass obscene amounts of wealth.
Remember that people in the magical world come of age one year earlier than muggles. I do think some wealth was inherited, but I also suspect James was something of an adventurer (remember his outings with Moony, Padfoot, and Wormtail), maybe Lily as well (they were both Phoenix Order members), which would have given opportunities to come across gold and rare items like the invisibility cloak.
Why would someone downvote my comment above? I did agree a source of the wealth was likely inheritance. But why couldn't James, with his clever, some would even say mean (to Snape), talents not also be likely to go out and get into adventures, take a few risks? (before Harry arrived, of course) He was still young after all. Remember, Sirius was the wealthy one in school. James went to stay over his house as a kid. Nothing was ever said about James Potter having a significantly wealthy family. Also, Hagrid says "D'yeh think yer parents didn't leave yeh anything?" That sounds like Lily and James made at least some of the money.
