Go us!
You get charged once when you upgrade, and once per hour. So it's bad to twiddle the settings between low and high upgrade levels (like I did at our first keep) because you're charged once up front.
The RPs you get are 1000 * KeepDifficulty per hour. Homeland keeps are all diff 1 except merchant keep, which is diff 2. Enemy keeps are diff 2 for merchant, 3 for the two nearest keeps to portal, 4 for the two further away, and 5 for the keeps in the relic fort zones. The keep we held was a diff 3, and I chose that because I figured it would be among the last to go down.
I upgraded it to level 3 and left it there. The GBPs you spend are (50 * upgrade) per hour. In my experience the upgrades to level 10 are noticeable for the lord and patrols, but not the front gate guards that get pulled before a siege.