It's only possible to get the amount they donated at time of donation. There are special substitutions for this that integrations can utilize. So for example you can push the value to a field using CBSubs Fields integration. The substitutions, with examples, are explained under the Integrations > Options > Display tab at the bottom.

I'm not sure why you need CBSubs Options for this though. Donation plans already have price selection built in. You can push the amount they donated to a field using CBSubs Fields for donation plans with the following.

Integrations > CB Fields
Field: FIELD_HERE
Operator: Add: Field = Field + Value
Value: [TOTAL_PRICE]
Remove value on plan deactivation: No

I recommend using a Decimal Number fieldtype. This should add to the fields value anytime they donate through the donation plan.

That usage is working perfectly fine in my tests. The payment substitutions pull directly from the basket so [TOTAL_PRICE] will always return the amount paid as it's the mc_gross value of the basket.

My best guess is you've something else acting on subscription status change causing a user store to override whatever CBSubs Field integration has set. This can happen if for example you've an auto action acting on onCPayUserStateChange and you didn't enable Reload User under the Parameters tab.

usergroup changers

That will trigger a user store. If they're acting on onCPayUserStateChange try turning on Reload User under Parameters and see if your issue persists.