What with the new-look PlayStation Plus, it looks to be a bit harder than ever to get a really cheap PS Plus subscription or to save money with PS Plus card retailers. In fact, said cards for subscriptions seem to have disappeared almost entirely and there is a shift to wallet top-ups to then pay for your chosen tier rather than paying for it directly.
This means the best way to get PlayStation Plus cheap is to decide which tier is best for you and then, if your budget allows, go for a longer-term subscription. The 12-month subs of any tier will always offer you better bang for buck value.