gifting games after sale is over ?

Science will work it out, if nothing else :)

The amount of CV you get is always the full retail price of the game on the Steam store. Sale price doesn't affect CV unless it drops the price low enough to be added to the Bundle List.

For example, if "SuperAmazingGame" is normally $20, but is on sale this week for $5, you'll get $20 CV for it unless it's been bundled or ~95% or more off in the past.

There are a few exceptions - for instance, when the Daedalic games package is on sale.

Edit: if the normal retail price on Steam drops (if SuperAmazingGame's base price drops) at any time, your CV will drop along with it.

cv stays the same unless one of the following happens:
1- Game gets/were bundled before.
2- Game Max price gets lowered.
3- Game is removed from steam ( results in 0 CV for it ).
4- Pack/Bundle of games get a big discount which results in the bundle and any game in it to give 0 or very low cv until sale is over.
I don't think sales will affect the cv unless its a heavy one which results in a game getting added to bundle list.
The once above are based on what i experienced so might not be 100% correct. If anyone knows extra details or edits reply to this comment and ill edit it for OP and anyone else who might be browsing for the same reason.

steamgifts will consider a game bundled when it's discounted ~95% in any region. usually being 80-85% off in ROW regions is enough to bundle it since it will reach -95% in russia or mexico/brazil (depends on curency value so it will fluctuate).
also when it's in a tier 1-2 bundle site, but you already know that.

so your best bet is to buy discounted games that don't go over -75%.
you could also install enhanced steam to check regional prices, or use steam.db and compare prices there.

Thank you every one for the info.

history wise, you can check this: https://www.steamgifts.com/bundle-games

and further Usefull stuff: https://www.steamgifts.com/discussion/dsq6C/discussions-that-should-never-go-away-aka-sg-bookmarks