Chris, I quite like this domain - and think it is definitely saleable.
As to value, that's a hard one.
What I'd do is let the market tell you what it is worth.
I just put "cheap seo" into Google and their are squillions of people advertising with that term.
So why don't you try an email to 20 or 30 of these people? Try a novel approach perhaps:
"I have this domain name cheapseo.com.au that I acquired at auction some time ago. However things have changed and I need to sell it.
So I put the term into Google, and saw that there are heaps of SEO providers using this term as a key word. So I figured it must have some sort of value to a professional SEO expert.
The trouble is I don't know what it is worth. So I thought I'd ask the people that are in the industry.
May I please ask what you think it could be worth?
Or would you possibly be interested yourself - or do you someone who may be?
I'd appreciate any help or advice you could give me please".