I don't know what you thing about that. I suggest that the cost of buy back decrease in time.
For example : you are level 25, it cost 1400 golds to buy back but you don't have them... you have 1000... cost of buy back should be based on the coldown to respawn so when it comes to 1000 gold you can do it. When you have the 1400 gold but you waited 30 seconds to get them, it's a shame to pay "full price". Maybe I am wrong with that and it will unbalanced the games but I think it's fair because you already "paid" in time.
Give you opinion and sorry if the explanation is not very clear
It probably affect the normal gameplay. For example, you would accelerate your respawn in early lvls (spending little money, such as 1-200 gold) to get to your lane faster. Also, remember that there is a particular item in DotA (Bloodstone, and a very expensive one BTW) that let you reduce your respawn time. So, the game actually IS designed for you to suffer through that time xD