Welcome to FT... the key point is obviously what you personally consider the cash value of Avios to be. In general that will be somewhat lower than the routine purchase price from BA which is 1.67p/Avios outside special offer periods, but exactly how much lower depends on how you plan to redeem. I think I'm right in saying that most collectors would "buy" Avios at 0.5p without any hesitation, but many would become less interested at some point before the 1p mark.

In this case the Santander card gives you a straightforward comparator to judge how much you'd be paying, so it's pretty much your call at that point. Of course the availability of 2 for 1 vouchers, special redemption periods where you get extra Avios on conversion of Clubcard points... and so on and so on should all be factored in. Plus sign-up bonuses will make a difference.

This post and the following series are a good guide to what you should think about:

http://www.headforpoints.com/2013/05...s-credit-card/

And this post addresses (Raffles' words) "the vexing and unanswerable question" of how to value an Avios:

http://www.headforpoints.com/2012/12...irmiles-worth/

Enjoy.