I was told by a holistic vet years ago to use human-grade fish oil. I am not brand-loyal, but I do stick to major reputable brands (and Costco's store brand, which was well-reviewed by Consumer Reports).
I think the Kirkland brand at Costco is the cheapest, but I no longer live near a Costco. It does cost a little more from Amazon. Like others I also watch for BOGO sales, and try to match it up with a coupon. (Same with Ester-C, which I buy for my dogs -- wait for a BOGO sale, and find an online printable coupon!)
So...Nature Made is BOGO this week at CVS (through Sat. 1/4). Also, at Target, the same brand is $5 this week (again, through Sat. 1/4). If you happen to get All You magazine (sold only at Walmart, by the cash register), there's a $2 off Nature Made coupon in the current issue (or you can find $1 off on the mfr website if you sign up for the Nature Made Rewards program, I think).
If you buy Nature Made fish oil, be sure to log the reward codes in their rewards program online, as they'll eventually add up for enough points for a $7 coupon to be mailed to you. Hold that coupon for a BOGO sale, and you'll likely get two big bottles for nearly free.