I just recently (2 days ago) bought a new mattress, so I guess I should post. First note that I was sleeping on a twin from childhood, and while I was surviving despite the massive dip in the middle of the mattress I feel that it killed my sleep during finals (when you are actually trying to sleep) and my gf is coming in a week and we have never used anything smaller than a queen in the past.
I think craigslist is the key. What you are looking for is NOT A USED MATTRESS, you are looking for a mattress wholesaler or similar who is attempting to get rid of some extra mattresses on craigslist. The difference of course is that these are new mattresses still in the plastic, but you will get them for the same price as people offering "clean/barely used" mattresses on craigslist (as if there was such a thing).
When you see an ad like this "mattress new in the plastic, full warranty, must move soon, etc.," these are the steps you should take:
1 - Ask the model number/year (this is tricky since there is a massive subterfuge effort when it comes to model names so you can't shop around, many mattresses that are the same use different names, but generally you can find it online or in a local store SOMEWHERE.
2 - Call up local mattress stores and say "Hi, my recent <above mattress model> somehow died <be creative>, I loved it and am looking for another." Now they will either say yes/no or they will say "well we have a serta XXX which is the same mattress as the <above mattress model>, but made specifically for our retailer. Generally this is the truth, especially if you make clear that you have previously owned it, they are full of shit, but they aren't that reckless.
3 - Check it out locally, lay down on it, see the suggested retail and their sale price (which will look amazing as its 40% of the manufacturers suggested retail), resist buying the mattress there as mattresses are absurdly marked up and generally mattresses of the same type (spring, etc.) are very similar, although the exact pattern of springs on each mattress is slightly different to avoid anyone actually doing a classification of mattress types (once again subterfuge).
4 - If it seems legit locally, head to the craigslist guy and pick it up (take normal Craigslist precautions of course). I bought a mattress, boxspring, rolling frame + a delivery (saved me the truck rental) for less than 400, including me tipping the guy. The mattress retailed for some ungodly amount, and the sale price at the local retailer was twice what I bought it for. Clearly I could have gotten a cheaper mattress, but this is a legit queen mattress+boxspring+frame for the price of an ikea mattress (which aren't actually as horrible as you think).
I think most people will agree that it is worth it to get a new mattress. I don't know how me+gf sold our college ikea bed+mattress off, but I guess it was to other college students. In any case, my sleep schedule had been horrible for weeks. I couldn't get to sleep until like 4 in the morning even laying in the dark on my old bed. First night I was out at 12 and woke up at 7, I almost cried from happy.
BTW, I know you said "spare no expense," but there is never a reason to avoid saving some money with minimal effort.