Sir the concept of declared value is done away with and now it's only on value of supply. So it depends on how much you actually receive from customer. According to that the tax slab is to be ascertained. If it is 2200 you receive, then 12%
As per Noti. 13/2018 CT (R), Gst is applicable on Trasaction Value , not on declare Tariff, hence the transaction value is 2200/- (3000-800) the GST is payable @ 12℅ on 2200/-