Say if I were to purchase something on
somewebsite.com using PayPal. If
somewebsite.com was compromised, what are the chances of my PayPal account being hacked compared to before the breach?
With a proper setup the Paypal credentials itself are not known by the shop so they should be safe if the hack occurred after you've bought something. But some data like shipping address or email might be shared with the shop by Paypal and thus can be compromised. Also, if you enter your credentials after the shop was hacked it might be that the attacker changed the site so that it now grabs your credentials, i.e. similar to what many sites do which are involved in phishing.