Why Does The Queen Never Change Her Clothes?

I agree, King Minos was always seen wearing the same blue robes, so I'm wiling to bet there not the same outfit, its just there either traditional or ceremonial and they have several pairs. Don't forget the Monarch also has to do religious ceremonies as well.

It definitely has to do with the budget, but surely it's an easy fix.
The BBC has access to lots of sets and costumes from their other television shows, surely they could use some clothes/jewellery from some of their other BBC shows? It's all about money, notice how most of the big battle scenes are at night, instead of during the day, so they don't have to do lots of big, wide shots that cost lots of money.

Many of the major expensive scenes are at nighttime/during darkness (notice how a lot of the monster scenes are at night? Whenever they discover something, or a new place, etc, it's almost ALWAYS at night. For example: The ugly sisters, the skeletons, the zombies all appeared at night, Pasiphaes attack on the city, the gang getting lost in Necropolis, the 3 guys being chased by the one-eyed giant and the royal convoy being attacked on the way to Aegina all happened at night) so that the special effects/CGI costs are kept to a minimum by using more simple techniques that are less noticeable under the cover of darkness.

All little tricks employed to save money.
However, the costume thing is just too obvious and distracts from the, otherwise great, show.
Even if they got some fabric from a thrift store! I'm sure those talented costume designers at the BBC could run up some dresses for Ariadne and the Evil (former) Queen for next to nothing. It's a silly oversight and I hope they rectify it. Don't even get me started on Jason's ridiculously short trousers that he's had from day one lol... I mean, damn... we get it, he's poor, but the guy can have at least two pairs of trousers can't he? lol... Jeez... And the perfectly, perfect, always sees the good in everyone and everything, pure as the driven snow Jason thing is getting a little stale now. Dude is BORING, he's humourless, he's ONE DIMENSIONAL. The writers need to switch it up a bit, ya know?
The musical score and supporting characters are great though.

It's a good show, could be great, but the aforementioned bad points stop that from happening. I really hope they tweak a few things to improve it.

If memory serves Morgana on "Merlin" didn't have a whole lot of wardrobe changes either. I seem to recall a dark green dress she wore a lot or something. I remember something always standing out. Seems like this is kind of the thing for BBC fantasy shows.