Mono is not quite .NET, no WPF for example (which is a big deal imo).
COBOL is typically outsourced to India, which has pretty much killed the market elsewhere. Though, you may get super high rates if you can wait for the Y10K projects ;)
Then who maintains India's COBOL programs?
Seriously, though, it depends on where you live. Not everyone is as eager to outsource as the US.
hmm..ya...there was a mini-gold rush in Taiwan last year because of the Y1K problem (their calendar started in 1911). It was a hard market to break into since the software is in Chinese.