Well I have a 128k subs Music channel. Also I think growing @sing to 500k +followers shouldnt be too hard. What I thought is that you could sell shoutouts by posting good Music covers etc from other people later on. I think it shouldnt be too hard getting 4k back. Dont you think so?The prices are high based on the usernames obviously not based on the followers!
If you can afford it and you have a solid plan to generate money every month from one of them then i'd get the one you want. The @sing account would be better than @Cover for music purposes forsure... I would see if the seller would do like 4-5k for the @sing if I were you and @Cover is actually a decent price for the name you get.
I notice many people say something is "to expensive" or "overpriced" when THEY cannot afford it but if you have the money OP and have a solid plan than you should definitely get one of them!