A standard pack of Dragonshield sleeves usually runs you around 10€/pack at retail regardless of what kind of sleeves they are. Some colours are more expensive, art sleeves are also more expensive usually. Matte clear sleeves, depending on where you buy them, either go for the 5-6€/pack you paid online or they’re at around 10€/pack at retail or local game stores/trading card shops. So you paid a good price for yours. And considering that you’re buying quality sleeves that are very durable and will likely last you quite a while, that’s a good deal.
Basic boardgame sleeves are usually cheaper but they’re also heaps cheaper in terms of quality, don’t shuffle well, are very flimsy and break easily. If you don’t mind that, you could have also gone for those. Not a huge recommendation on my part and since you’re already decked out too, probably not needed at this point.
Overall, it’s not cheap as an absolute number, but it’s comparatively cheap for what you’re getting and how much you could have been spending for them.
You could look into 3D-printing a new box or getting a wooden box off Etsy or something if you find the cards not fitting anymore. If you find one in the size you need