First and foremost, I would say if you have the extra money to spend, spend it on your diet first. This is going to be the best source of those vitamins and minerals. You will absorb them better from fruit and veg than you would do from a supplement. Then, if you can spend more on a supplement once your diet is sorted and you want to, then do so. Think of supplements as a safety net.
It isn’t as straightforward as saying expensive is better and cheaper ones are rubbish. If you’re looking to get just folic acid and vitamin D, there is nothing wrong with choosing a cheaper option. They will still provide you with the nutrient you need.
But it is true that the amount stated on the pack can vary quite substantially. It can be 20% either side of what is actually stated. In particular, some things are added to help with shelf life. Some more expensive supplements might have a shorter shelf life. With a shorter shelf life, you might be able to assume they’re more closely matching what’s on the label. With a longer shelf life, they may have put more in to account for a decrease over time.
So many different things affect how much you absorb from supplements. Broadly speaking, you should aim to get most of the nutrients you need from your diet.