I wouldn't go as far to say almost every club in the Championship would consider a £500K-£1M fee as a 'drop in the water'. Especially with the amount of Championship clubs that are slashing their budgets this season. Championship clubs will spend such fees on a more than regular basis, something that I'm not contesting I might add, but if you look at the bottom half of the table, the quality of players and the size of fees clubs spend are more or less worlds apart from the top half of the table. As is the norm for probably the majority of the leagues throughout the world. For instance, if you look at the expenditure of some of the lower placed clubs in the division: QPR, Preston, Sheffield Wednesday, Birmingham City, Hull, Rotherham, Bolton, Ipswich didn't spend more than £3M each on transfer fees during the summer window. £500K - £1M on a player unproven even in the SPL certainly wouldn't be a 'drop in the water'.
Anyway, my argument is surely clubs such as those you have mentioned (e.g. Millwall, Wigan, Rotherham), as well as some of the above are more likely to target Shankland because he fits within their budget for a goalscorer. Whereas realistically IMO, are higher placed Championship clubs aren't interested in the likes of Shankland due to the depth they already possess in those positions, and to put it simply, their own shear snobbery? To put it into perspective Blackburn brought in Ben Brereton earlier this month for £7M and he's more of less a bit part player. Whereas Preston shelled out £300K to bring in Burke from Shamrock (Example of a lower end Championship club taking a chance on a young prospect from a lesser league). Brereton is the kind of player these clubs bring in to remain competitive. I understand your point thought as in theory of course for the likes of Villa, Leeds, Nottingham Forest, buying Shankland would be a drop in the water, and why wouldn't they take a gamble on someone like Shankland given his record this season. But IMO it's just not as simple as that as managers at the top end of the Championship are massively under pressure to achieve promotion and believe it or not still have a budget they're restricted to. Slightly creating a scenario here but who's to say that extra £500K - £1M in their budget could just be enough to get a 'signing of the season' deal over the line so why would they be interested in a Shankland when he'll probably never play for them.
I agree with you on that, outside those you just mentioned they probably couldn't. But that's not to say they could if funds were made available due to a later sale... This scenario reminds me of when Jordan Rhodes had that ridiculous season where he scored around 40 goals for Huddersfield and he was linked with a host of Premier League clubs. Where did he end up...Blackburn. I could be completely wrong and come July 1st he's signed for Middlesbrough or Villa. But IMO do I reiterate, I'm sceptical about how many ‘top Championship clubs’ are actually going to be genuinely after Shankland...
