iMovie costs money now?

Solution 1:

I had a similar experience.

iMovie came with my MacBook Pro when I bought it in 2011, but I could not update it in 2014.

I contacted Apple Support, who checked the MacBook ID and restored the purchase.

Solution 2:

Go to App Store > Purchases and click the cloud icon next to iMovie (or any other app that you would wish to download) to download it again for free. If for any reason iMovie is not present in Purchases, then, unfortunately you will be required to buy it again.