Guild Wars 2

Bad at math here.

How do you guys figure out what is the best option to get extra things like bag space and bank space and character slots?

For example, right now I can get:

100 gems per 3g 21s
or
25.00 for 2000 gem card @ amazon
or
10.00 for 800 gems, 20.00 for 1600 gems, 35.00 for 2800 gems.
or
60g for 1938 gems

I really am lost when it comes to figuring this stuff out.

I buy gems if I have extra money and am too lazy to earn enough gold to make the purchases. So that’s your choice – do more in game and earn the gold or decide to spend your money if you think that you’ll get your moneys worth for it.
Now when it comes to gold:
Try to not trade gold for gems right after a new item came out. Wait 4 or 5 days. The price falls then (right now it’s not that high)
When it comes to money:
Browse various gem card sellers and find one that sells for least, then find the gem amount that’s the closest to how many you need (that way you won’t end up wasting money).

Gems are always 80 for 1 USD or Euro

Gold price for Gems vary based on players buying and selling gems for gold.

The question that only you can answer is right now what is easiest for you, buying gems with gold or with actual currency. If you have 100s of gold, earning gold is easy for you and if you can part with some of it then great, pay with gold. If you have some extra cash in your pocket you can part with, buy gems with it.

It’s all up to you.