Semporna, Borneo for diving in Sipidan and surrounding islands - I don't remember the costs but its a small fishing town so if you stay there and not on one of the expensive resorts on Mabul or Kapali be aware that there isn't alot going on for tourists, but the diving makes up for it.
And fresh mango's from the market (buy first thing) are great for surface intervals (just remember your knife)
Personally I'm a budget traveller because I love the freedom backpacking can give you if you have the time and the inclination rather than "package holidays" offerings of a country, which doesn't mean I'm opposed to a spot of luxury now and then!