Best way to update product price without updating past deals?

So I have two board this question relates to. “Products” and “Deals”. If we have a bunch of deals, say 100 of existing previous deals and we decide 2 or 3 years down the line we update the pricing. From say $200 to $300. What is a way to affect the new deal prices without updating to past deals?

Currently, I used a formula that would copy the mirror “unit price” from the product board when a product was selected and it would have several formula columns that would run automatically and so far work great… except for one flaw. If I update the product’s until price then every past deal updates automatically.

So far I used a general caster 3rd party app auto task to cast the value when the product is selected it will cast the “deal value” which is a formula caster automation that calculates “quantity” times “unit price”. However, if I update the quantity I have to re-cast again and this eats up a lot of 3rd party task automation very quickly

One alternative I thought of was what if I set up a automation task where something like “if ‘product’ is selected then set ‘X’ value for ‘unit price’” and just have a column that isn’t mirrored called “unit price” and then in the future if we want to update pricing we can just change the value so that way in the future if we update pricing we just change the value in the automation but it won’t recast or rerun automation long as product isn’t altered.

But is there a better way to do it that I haven’t thought of that is better than this? Maybe more simple that I’m over looking?