That question is about as open as "How much is a bike worth?" There are horses for courses. If you google Bike "bike panniers" you'll soon find something that will suit YOU. The variables include, size, quality, function, country of manufacture, etc,etc. here's a pic of mine that suits my touring real well.
The red/grey ones are Deuter and fit my MTB front or back. I have them mounted here on my recumbent. They cost $100au. The rack was another 100. The yellow set are the custom Optima bag set for my Lynxx. The rack was $110 and the bag set was $100 for the seat back bag and $350 for the large side bags. I do long trips and need to carry heaps. I also tow a trailer. These bags suit me and I consider I have good value for my money. You research google and you will find something that will suit you function-wise and $$-wise.
I hope this helps!