If you are not happy with what they are offering as compensation you can keep trying to negotiate with them for a better deal.
If they will not you can look elswehere for a better deal and cancel the contract as they have not been able to supply what they have agreed.
If you do not want to go elsewhere you can take the matter to the ombudsman and have them decide.
But I agree with you a reduction to 10% of what you initially entered to receive and a price drop of only $20.00 does not compute.
Particularly when for $80.00 you get 500GB according to their site.
I think a costs of $20 or $30 would be more appropriate.
I hope this makes sense and is of assistance. If you wish a phone call please elect for one but be aware it will be an additional fee. If there is nothing further thank you for using my services.
If I have missed anything, or you have any further questions please let me know
If there is anything else in the future please do not hesitate to ask.
Please do not forget to leave positive feedback.