We have 2 Goodwill stores in our little town of 35,000 people. One is old. The other is new. In the older store....which is much smaller and dirtier....small goods like toys, kitchen items, etc....are always dirty and priced like you are seeing. They price it like it was a new item rather than used, dirty, scratched, or otherwise garbage. The larger items such as furniture are priced pretty good. We've picked up some nice papasan chairs, shelves, card tables, etc...for dirt cheap. It's like whomever is pricing stuff has no idea what they are doing.
The newer Goodwill is ok. It is definitely cleaner and has a bigger selection. We've picked up some things there too like camping cots, stuff for Halloween, etc. Prices are reasonable but nothing to write home about.
Of course, you are doing a different type of seed tray....but what I ended up doing is going to the dollar store (where everything is $1). They have very cheap gardening stuff like planters, buckets, foam kneeling pad, cheap tools, etc. I just picked up what I needed from there and could re-use for gardening purposes. I raided the kitchen section too for stuff I could repurpose for the garden.