The cost approach is used in real estate to give value to new houses based on the value of the land and the price of the construction. It is said to provide a more accurate return on recently built houses. The approach can be also used for old properties but depreciation must be included in the calculation of the house value.
I bought an apartment with my boyfriend last year because we had been dating for 4 years and he proposed. In order to prepare, we saved up our money, asked the bank for their opinion on the best coarse of action financially, and we tried to decide how much of our savings we should use without being irresponsible. (This is just an example. I am 15 and will be forever alone but yea this is what I would do anyways)