In my South America trip I discovered the cheapest way on how to travel from Quito to Bogota overland. I bit less than 40 USD to cross from Quito to Bogota (7.5 USD in the Ecuador side + 90.000 COP (31 USD) in the Colombian side).
Quito to Tulcan by Bus
Take a bus or taxi to the Terminal Terrestre Carcelen in Quito. From there purchase a ticket to the city of Ipiales. Buses leaves every hour and it will cost you around 6.30 USD. I left at 9AM and arrived at 3PM.
Tulcan to Border by Taxi
The Taxi to the border cost 3.50 USD and you can share with fellow travelers. I shared with more 3 people and payed 1 USD. It is a 20 minutes ride.
Make a stop over to visit Las Lajas
Go trough immigration in Ecuador than cross the bridge by walk and stamp your enter in Colombia immigration. There is an amazing information center where you can get a map and all information you need about the country.
After I stamped my passport I was approached by a bus driver that asked me if I was going to Bogota. He explained me that a direct bus would cost me 130.000 COP but with a stopover in Cali would be much more economic. I told him I wanted to visit Las Lajas Sanctuary so he offered me an amazing deal: I bought a bus ticket with him in the bus office in the border to Cali for 40.000 COP. He payed my taxi to the terminal where I could get a shared taxi to Las Lajas for 2500 COP one way. I had one hour to see the church and head back to the terminal to take my bus. It is really worth to visit the church, build in n the Guáitara river canyon and a jewel of engineering.
Ipiales to Cali by Bus
I took the bus at 5PM in the bus terminal and arrived in Cali at 4AM from the next day for 40.000 COP.
Cali to Bogota by Bus
A bus to Bogota was available at 5AM and arrived 3PM in the capital of Colombia. I payed 50.000 pesos with the company Velotax. The ticket in the premium company Bolivariano was 80.000.