If you are able to afford and consume enough protein in a day through whole foods (i.e. without needing to supplement for whey), then I guess creatine, but you can get a year supply of that for like $15.
When I've had to live on my tightest budget, I think whey still came out to be the most important supp for me. If anything, having whey saved me some money. Cost of poultry is dependent on where you live, but for me, a generous scoop of whey was more cost-efficient than a couple of chicken breasts or cans of tuna.
Obviously still get as much of your protein through whole foods as you can, but if you have 40-50 g of whey a day that can end up saving you some money. Another supplement to look into that can actually save you money would be dextrose. It's not the ideal carb source, but it is simple, works, and is cheap as heck. Again, whole foods are preferable, but if you are on that tight of a budget, the use of whey and dextrose can help you save $$, have decent peri/post workout nutrition, and still meet your macros.
As your research has yielded, fish oil is good, too. Since dextrose and whey would technically be saving you money (operating under the premise that you have specific macro goals) and since creatine is dirt ass cheap, maybe you would be able to squeeze fish oil into your budget. How badly you need it/how noticeable it would be is dependent on your goals, but definitely everyone can benefit from it.