Can i keep like 6 months or more of backups available for restore? is that configurable?

Yes. You can configure the retention required for your backup jobs. The retention can be configured in the following 3 ways:

By backup age (default)

Specify how long to keep backups created by the backup plan.
E.g. 8 days for daily backup, 5 weeks for Weekly backup, 13 months for Monthly backup.
By default, the retention rules are specified for each backup set separately. If you want to use a single rule for all backups, click Switch to single rule for all backup sets.

By number of backups

Specify the maximum number of backups to keep.
E.g. always keep 10 backups so that there will be 10x restore points at all time.

Keep backups indefinitely

As described, your backup data will be stored indefinitely.

In terms of pricing, depending on your preferred licensing option, you will be charged based on the backup data capacity that is being store in Cloud Storage. Or based on the quantity of devices/seats being backup.