I think it is slightly unfair to base the price of vouchers in terms of pebbles, based on the pebble to moonstone ratio which isn't set in stone, but rather a result of the community economy. I do agree however, that apart from the shop exclusive items, vouchers pretty quickly lose their value and use. I already got almost all the companions from the weekly shops, and don't really have anything else to spend it on, making them pretty useless (even without the comparison to moonstone - pebble ratios)
But because the moonstone - pebble ratio fluctuates, and is currently the result of a, in my opinion, very flawed system where moonstones are too difficult to obtain and pebbles have very little use, I don't think changing the voucher prices of items that can also be bought for pebbles based on the current moonstone : pebble ratio is the way to go.
Instead, we need good pebble sinks, we need more ways to obtain moonstones, and I think the shop could do with some extra content. The site is still relatively new so I am sure the devs are already planning the expand the shops' inventories with more items over time, but knuckelbone tokens for example are a very easy addition to the weekly shops (I actually made a suggestion about this just a day or two ago).
I also think that vouchers shouldn't have such a hard daily/weekly limit. I would love it if whatever activity is being celebrated for the week also still occasionally/rarely has a chance to award additional tickets. Making them easier to obtain also makes it easier to spend them.
And lastly, I think the vouchers shouldn't be accountbound. Accountbounding so many items completely stifles the economy, and the economy already isn't in a really good spot due to many other reasons (including the ones I mentioned above).