For me its impossible to quantify how much FS2004 is worth for me. The amount of money invested in payware doesn't say much because for FS2004 there was far more freeware available than there is for for instance FSX. And the freeware offered was often from at least the same quality as the payware in those day.
And also the amount of money asked for payware was much lower. The WWII fighters package by Shockwave/A2A is a fine example. It was a CD with 5 different aircraft, some even with multiple versions, it came with free updates, which could even turn models into a FSX native version for those who flew the newer sim. I bought the CD for 25 Euros, which means less than 5 Euro per model. The models were amazing for those day.
As FSX offers more possibilities the models are more complex and need more develop time and also the customers have become far more demanding, it is logical FSX model are more expensive and therefore a single model for 25 Euros is already a bargain.
As Priller already indicated its a hobby and FS2004 can be altered into something really beautiful, just by using freeware. For me that is a large part of the fun. When I compare the amount of hours I actually fly with the hours I'm tweaking the sim, it becomes very clear that I actually can't call myself a virtual pilot. FS2004 is reasonably simple so making your own mods like repaints is still reasonably simple as well. So in a short time you can achieve very nice things.
I can sit watch all the things happening at Maschy's Gutersloh for hours. Its all free, so it doesn't really has a value which you can quantify in money. But the fun it brings just to watch is more worth to me than the value in money of many of the payware add-ons I bought for FSX.
In my simple dictionary value is the amount of money somebody is willing to pay for something. Therefore FS2004 is priceless for me........
Cheers,
Huub