I think there are a number of factors that need to be considered:
1 - The length of time you'll be using the lounge - $50 for QC or Business Lounge (BL) or $100 for First Lounge (FL) is a lot of money if you only have 30 minutes to enjoy the lounge. However, the value starts to add up as you go past the 1 hour visit, and would be great value in my opinion if you've got 1.5+ hours to enjoy it, especially the FL
2 - whether you like to drink alcohol - I know that when I'm not flying to get to a work meeting (for city breaks, holidays, etc), I'll want to get into the spirit at the airport, and for me this means having a few tipples! As we all know, beers, wine etc is incredibly expensive at airports. Access to a lounge for 1 - 1.5 hours allows you time to kick back and have a few, otherwise expensive, beverages. This value factor isn't going to be applicable to morning flights though - I don't think you can drink anywhere near as much coffee as cold beer!;
3 - whether you have company - If I'm traveling with my partner or a friend, the social aspect of lounge access is heightened. Sydney Domestic Airport public bars and cafe are notoriously over crowded, with dirty tables common place. When with company, it's great to be able to relax in a lounge (time permitting), share a meal and a few drinks. In my view, in can replace a perfectly good cafe/pub/restaurant meet up. The value of this factor increases significantly for FL for me.
Keep in mind that most of the passes available for purchase on eBay, Gumtree admit one person only. So you'd need to buy two. In my opinion, $100 for access for 2 people to QC, BL for 1.5 hours is a great deal, for the reasons above. $200 for access for 2 people to FL is a lot of money, but no doubt the food, service and quality of wine is much better. As a person who doesn't mind spending a bit for good food and great wine, the $200 can be extremely reasonable, particularly if it's the start of a holiday trip (and you can get the extra benefit of number 5 below)!
4 - your personality! - If you couldn't give a toss about overcrowded spaces in the airport, don't care for fancy food or nice wine, don't need some semblance of relaxation during your airport visit, or are the type of person that finds themselves running through security checks to board your flight in 30 minutes - then don't bother paying for lounge access. However, if like me, you're an aviation enthusiast (and that extends to plane spotting, airport etiquette and people 'who are going on holiday' watching) then it's worth considering. On balance, I'd be more comfortable forking out $50 for BL than just QC.
5 - did someone say massage? - In Qantas FL in Sydney and Melbourne, you can usually get an appointment with a masseuse in the FL spa. Priority is given to Qantas First Class passengers of course, who get to make booking the day before. For everyone else in the FL, it's a rock-up-and-see-if-there's-a-slot service. It's about a 20-30 minute massage or relaxation therapy, which was great for me on the 2 times I received it. I get the impression from online reviews that it's not difficult to get a spot, particularly if you've got 1.5+ hours in the FL and can be flexible with time. You won't know whether you can get a massage until you get to FL, so it's hard to include it as definite value in the $100 estimated cost of a pass - but if do get one, then wow, you've got your value! Finally, keep in mind that massages are only available when the FL spa is open, I think that's 8.00am - 4.30pm in both Syd and Melb FL.
If you're considering it - my advice is to just go for it. Spend $50 on BL access if you can, go test it out and see if it suits your personal desires/expectations. If it doesn't, well you've only lost $50 less the price of a bit of food and some drinks (at least get one!), but you have resolved the ever burning question inside of you. If it does meet you desires, great - you've discovered something amazing to enjoy for the rest of your Qantas-related traveling life!