In total I spent about £600 in Mongolia which was significantly more than I expected. Having said that, I stayed for 5 weeks and travelled about 5,000km, and had some absolutely fantastic experiences. A rough guide to what I spent is below.
I had to extend my free 30 day visa and pay £35 for an additional 30 days. We spent £60 (and 3.5 days) each travelling from Ulaanbaatar to Bayan Olgii and back by bus. We paid Dorsjan, a guide from Kazakh tours, £75 each to arrange our transport and stay with Ardack. We then paid Ardack and his family £3.50 per night to stay with them, and about £15 each for some vodka, sweets, socks and fuel for them. In total the trip cost me £175.
I then paid £20 for a return journey to Moron in the north. Here, I paid Saraa £150 for my visit to the Tsaatan people. This included public transport each way, a horse and guide to take us to the mountains and back again and a border pass. I then paid for food and another £15 for accommodation with the Tsaatan people. In total the trip cost me almost £200.
These two trips were quite expensive but, in my opinion, worth it. The rest of my money went on transport (a couple of short taxis and buses etc), accommodation and food.
Sharing hotel rooms with Jordi kept the costs down as hotels often charged by the room, so we could pay half each. On the flip side, travelling in the off season means that cheaper accommodation options are no longer available. In total I think I spent £120 on accommodation.
The last £100 I spent mostly on food. We often ate out as we did not always have cooking facilities but we did not eat extravagantly. Normally we spent about £2-3 each on a meal, often even less. The most expensive meal we paid for pizza, for about £8 each. It is only Western food that is expensive though. Both being coffee lovers, we did treat ourselves to a couple of expensive espressos after 15 days without coffee! They were good.