To get deeper into Kolbiorn Barrow, you'll need to continuously give Ralis more money as he asks for it. You need to do this four times, starting with 1000 Septims and ending with 5000. In total, the excavation of Kolbiorn will cost you 11,000 gold. Each time you pay him, leave the area and wait for a courier to approach you with a letter from Ralis. Each time you get a letter, return to the excavation camp and clear out the Barrow. Each time you return, more of the barrow will be available.