I guess you already have a variable for coins. What you have to do now, is this: one way is to use modulo. I think that's the simplest, because you don't need any additional variables.
Add an event:
System|Compare two values: Coin%100 = 0 (and) Trigger once while true -> System|Add 1 to Life
(Assuming Coin is the variable for coins and Life for lifes)
https://www.dropbox.com/s/yyaxf6pw0emqt ... o.jpg?dl=0
Maybe you may want to add another condition: Coin /= 0, so 1 life isn't added on start when you have 0 coins.
https://www.dropbox.com/s/gyhd4vxhz3u6y ... 2.jpg?dl=0
What is modulo?
Modulo is actually the remain of dividing. In C2, it is represented with percent sign (%).
50%100 = 50 (50/100 = 0, 50 remains)
100%100 = 0 (100/100 = 1, 0 remains)
200%100 = 0 (200/100 = 2, 0 remains)