Personally, I think it would be more trouble than it is worth to try and make your own herb mix that is identical to Herb Salad. It contains 22 different herbs, some of them being rather hard to find for sale on their own. Buying all those herbs in bulk and mixing them together yourself would likely cost you as much or more than reaching out to the makers of Herb Salad and finding out if you can buy their product in bulk. Plus, if you source your own herbs, you'll want to be sure to double-check all sources to ensure the herbs are bird-safe and pesticide-free. This will likely limit your options and raise the price. You can certainly look into it and could probably make a more affordable alternative by cutting out some of the more expensive or harder to find components, but at that point it isn't the same product. You could offer the less expensive dry herb blend some days and Herb Salad on other days to cut down the cost while keeping the benefits of the more expensive mix.
If you have a green thumb, you might try growing your own herbs, so you can supplement your supply of dry herbs with some fresh offerings. This would cost more time and space, but less money.