I'd take a punt on him on low wages (no more than a 50% increase on whatever motherwell are paying him, supplemented by goal bonuses), preferably signing him to a pre-contract as soon as possible, then offering motherwell maximum 100k for early release.
Best case, we get him, worst case, we've unsettled their star man for the rest of the season.
Same approach for McLean and Walker too.
I don't think we should be offering massive wages to SPL players anymore - the story about Nicky Clark entering negotiations with thoughts of (x) amount as his "aiming high" figure while expecting significantly less, only for Ally to offer nearly double that springs to mind. Also Kevin Kyle asking for what he considered an outrageous wage and being given it, Ian f'n Black on 7k, Sandaza's supposed structured deal that would have had him on something like 10k when we returned to the top flight...
We offer sensible wages, still higher than the existing contracts, and top them up with performance related bonuses. If it works out, the player gets more cash, if not, they get their downside guarantee, and the club are no longer raped by charlatans purely because of who we are.