I did something silly last night, was progressing well and had got up to 180 so decided to get the Legendary weapon & Armour mods to bump up my power level so I bought a shitload of blue mods, exchange them for purple ones & left myself with about 5,000 glimmer. So I go to add all my newly acquired purple mods and....
I completely forgot that it costs you glimmer to add any sort of customization to weapons and armour after the fact that you have them in your inventory. I only had enough to add one purple mod and the rest of the night was spent glimmer farming and doing the doable milestones. In the end I got there though and am now at 187.