$30 more, plus you have to buy through a no-name Amazon seller. Maybe other people are okay with that, but I feel much more secure buying from Google directly (in terms of whether or not you feel secure having a Google device is a whole other story, of course...).
Checking on best buy (as a US user), the A71 actually goes for $600 if you want it unlocked and without activation [1]. So a more fair comparison is that for Americans the A71 is $250 more than the 4a, but you could get it for cheaper if you activate or buy it locked, or are willing to buy from a 3rd party on Amazon.
The A51 does seem to be available from Best Buy for $400, unlocked and unactivated, or $300 from an Amazon 3rd party, but the specs there are of course not as good as the a71.
Posting as I was excited to buy something non-Google and fully featured, but I personally don't feel the same confidence buying from some 3rd party on Amazon as I would from Best Buy or Google.
1) https://www.bestbuy.com/site/samsung-galaxy-a71-5g-128gb-unl...