According to the League of Legends wiki page, each percentage point of cooldown reduction is worth 24.75 gold, so 10% cdr would be worth 247.5 gold. Obviously, it's going to matter more in the early to mid-game where gold differences between teams is important, but the later the game goes on, the less that matters. Also, you may feel like you're wasting an item slot on an item that over-caps you on cdr, but if the item that contains that cdr is over 100% gold efficient, then it makes up for that gold spent on cdr.
For example: if you had 40% cdr already but you bought a Spirit Visage, you would be hurting your gold efficiency by over-capping on cdr. However, Spirit Visage is over 100% gold efficient so it makes up for the overcapping in cdr.