Here in the US, I regularly see 19" 720p tv/monitors on sale for less than $100. I'd think that would be a good solution for a development monitor as long as you don't require 1080p for some odd reason. The built in TV tuner would be a bonus.
A cheaper solution would be an old (recycled?) DVI monitor using an HDMI to DVI adaptor/cable. At work I've even seen old DVI monitors with dual inputs that you can switch between. The only downside there is no sound through HDMI, which isn't ideal if high audio quality is a requirement.
The cheapest solution by far would be an old TV with a composite input. I'd think most US households have one of those somewhere in the house. I've still got my 80's era TV/monitor I used to use with my C-64 which would fit the bill quite nicely. No the display isn't the greatest by today's standards, but it was certainly good enough back in the day.
Engineering solutions are often a compromise.
Ahhh, I look forward to when that under $100 is under $50 Maybe I should keep my eye on Craigslist.... but in my case, a big move came just months before the official change-over to digital so we just left the old analog TVs behind. But bringing them across country seemed pointless. And I totally agree on the compromise angle you mention. Unless the RPi is going to be specifically used for HD video viewing or high end gaming, I don't see the logic of connecting a low end computer like this to an HD screen. Even 720 seems like overkill. But hey, we all have our ideas for the RPi.