I thought I should mention while the RockShox Monarch air shock is a very good unit - there's a reason for the price discrepancy, why the same shock is offered with such a wide price range & why it's still usually less than other comparable branded shocks... During their initial production run they had difficulty with the Schraeder valves failing to seal properly, as it was only a minor leak & a popular brand the market was flooded with these shocks before they realized there was a problem and offered dealer compensation.
As I understand it there was both compensation (partial refund for distributors) & an exchange program and not a full-fledged recall as it was such a minor issue - as a result many of these "defective" shocks are still readily available, usually at a reduced price vs the later variants - but few distributors seem willing to mention why.
As I had a project that needed three quality dampening shocks, was unwilling to spend the $600-900 & was informed of this by a friend (a LBS owner), I purchased the "defective" RockShox Monarchs for about $100 per unit (unfortunately now out of stock) and simply added one tiny drop of sewing machine oil into each of the Schraeder valves which fixed the slow leak issue, apparently permanently as that was at least two years ago.
While I find myself equally disgruntled with shovel's observation that "There's a huge valley between department store sh%t bikes and serious bikes" & and a similar one with aftermarket parts - sadly that gulf exists and there's not much to be done about it, worse yet it seems primarily consumer driven in that they'll charge whatever the market will bear, regardless of what the item's value may actually be.
Despite my cynicism, I do wish you the best of luck in your quest - given the market and available shocks and after searching extensively for my own project, I would suggest the RockShox Monarch is most likely the "best" combination of comparatively reasonable price & function, the "$40" coil-overs are very often extremely low quality, costing as little as $12 before 3rd party retail markup... if you've patience, luck and are willing to take a chance, or just want to see what these retailers are actually spending on these low end accessories try
Alibaba.com ...but there's few willing to sell single units, no "dealer reputation" to speak of & if you hate ebay, you'll hate it more by the time you're done
I would agree w/the others that have suggested a rebuild or inquiring about take-offs (often new units removed for upgrades), it's true many bike shops charge premium prices for both labor & parts - but I've had great success and have saved untold amounts of money by skipping the sales people entirely and talking directly with the techs, many of whom are as "down to earth" as you'd expect, willing to talk honestly about products & might even take a "side job" at a greatly reduced prices - it's not always the case & many as clueless as their customers, but it's worth a shot...