I'm not (and I doubt most of us are) familiar with how big a difference that is at Air Canada. (I've flown Air Canada on an award, but it was booked with United miles, and only the lowest-tier Air Canada flights are visible to United.) The higher-priced tiers quickly get ridiculous over at Delta, which last time it published an award chart (it no longer does so), it had 5 tiers.

AA and UA also have most everything available for redemption, but only at two tiers (well, in AA case, 3 sub-tiers of the higher tier). The higher tier, generically known as Standard awards (but more clearly perhaps explained as "anytime" awards or "last seat' awards) typically starts at least at double what "saver" awards are.

You can find a separate recent thread about such high-priced awards here:
(So it would redundant IMHO to discuss that seperately here.)

However, even if someone is interested in those, those only ever apply to the airline's own metal AFAIK. Which often doesn't get you where you want to go in international travel. Partner awards pretty much always have to be available at the 'saver" rate to be available through partners at all.

Now, as to your question of miles + cash, it depends very much on how it's set up. I'm mainly familiar with it from hotel programs, where at some hotel programs (IHG and Choice) it's nothing more than buying points at the time of making the reservation (as the cash component is always the same for the same number of points that you're paying cash for), while at other hotels there's no such relationship and the value of points + cash tends to be good at mid-tier hotels for whatever reason, but not good at low tier or high tier hotels for whatever reason.

It is illegal for parties who don't have a contract with the airline to sell airline miles. In the US, even when airlines sell their own miles, it's invariable outsourced to points.com.

There is one very specific case of miles + cash that people would love, but I don't know how it would be possible: Use miles for the flight legs that are available on miles, use cash for (hopefully shorter connections) that aren't available on miles, yet have them all end upon the same ticket! It doesn't do much good to have them end up on separate tickets, unless you have TONS of buffer at the connection airports, because on separate tickets you are not "protected" if the arriving flight was late and the onward flight was on a totally separate ticket.