Hey El Cid,
$5,000 per month would give you an easy life in Panama (well most of Panama anyway). You can easily find a penthouse suite overlooking the Cinta Costera, or one of the large seaside homes in Coronado, and see that money dwindle away quickly. However, in most places in Panama, you'll find that you can rent a large house for $500 per month (closer to $1,500 and up for the higher end places like Coronado or hip city spots like Casco Viejo or Marbella).
AuntieFlo was right when she said it really depends on your lifestyle. Even then, I can't imagine $5,000 wouldn't be enough to live on. You can get to most places in this country by bus or taxi, but if you want to move around freely you'll want to have a car. So a big factor is do you already own a vehicle or will you be paying monthly.
I'd say the biggest factor is whether or not you're planning to rent or own a house. Without knowing your plans, and based on your message above, I think I'd look towards Altos del Maria, which is only about 45 minutes outside of Panama City (then up the mountain I think about 20-30 minutes). The views are stunning, the expats in the area seem to be friendly, and you can drive down the mountain and over to beaches and shopping of Coronado rather quickly. I met a couple who does this all the time. They love Altos del Maria. I found this listing in Altos for about $1,400 a month rent, but it doesn't seem to be the average. It might cost a little more to live there.
http://www.compreoalquile.com/consultas … iedad=NULL
Or you can just go to the Altos del Maria site and maybe one of their agents can help you find something to rent/buy. Check out: www.altosdelmaria.com. If I could afford to live there, that's probably where I'd hang my hat. El Valle is nice too and not too far from the beaches. That's the great thing about Panama. You can live in the mountains but super close to the beaches.
A lot of expats live in Boquete, and I know that's where AuntieFlo used to call home, but to me that's just too far away from Panama City. It's almost an 8 hour drive. Still, if you want to be around your countrymen, and up in the cool hills, you might even want to look that way. I'd rather find a happy medium along the Pan-American highway, where I can get quick access to the city and the beaches.
Hope this helps a little bit.
Chris
Oh, and check out my blog at thestayathomegringo.blogspot for all kinds of nuts & bolts info about living in Panama. And keep an eye out for my new site I'm launching in mid-April at [Moderated]