How much does a refueling stop for an Avro RJ85 cost in Bolivia/Colombia?
It's hard to give an exact figure as their are a number of considerations. If we assume that the re-fueling stop is at El Dorado, and the airplane was a BA146 (RJ85 is a variant) then approximate costs might be (in USD):


Landing fee: at about \$6 per thousand kg, and 42,000 kg would be about \$250. Add 5% for a night landing and another 5% for a night departure for about \$280
Handling charges: this is hard to say, estimate $100
Jet Fuel: the BA146 can carry up to 3000 US gallons, assume 1500 as it's a short hop, at \$3.16/gallon that's \$4740


So it would seem that about \$5000 is a good working figure. Keep in mind they wouldn't have used all that fuel to get to their destination, there would have been plenty left and they wouldn't have had to fill up completely on the way back, but the most expensive part of a flight is the climb to cruising altitude, so 2 climbs are more expensive than one. There might also be ATC fees for filing another flight plan, and taxes not listed, I would imagine that wouldn't change the costs that much - the fuel is still the big chunk of the cost.