In an average light water reactor, one gets about 500 GJ thermal energy out of 1 kg natural uranium (i.e. 3.5% U-235) on a once-through fuel cycle (i.e. without reprocessing), which means that you’d get about 430 GJ useable thermal energy per kg of U3O8 . It’ll be a bit more efficient (not sure by how much) if you enriched the U.
For comparison, a fast reactor and full fuel reprocessing would probably give between 20-50 times the amount of energy per kg, depending on the average number of reprocessing cycles.
Currently costs about €89 for that kilo of U3O8. Bear in mind that LWR nuclear plant is about 36-38% efficient – pretty run of the mill for thermal plant.