It's not very often that the decision is as simple as 'Do I want to spend X dollars to save this much weight?'. Usually there are a bunch of other considerations as well pertaining to the function, quality and usefulness of the item. With your rain gear example, the lighter rain gear is also likely to be less durable and it may be lacking features that your current one has. So the complete question may be "Do I want to spend X dollars and give up some durability and a few pockets to save Y ounces?". It's important to consider all the differences when deciding rather than getting starstruck at the ounces that can be saved. Saving weight is amazing, but you need to fully consider what you're getting into. I know I've wound up disappointed with a few pieces of gear when the expensive new UL item didn't work quite as well as it's predecessor.
For me, if it is a situation where there are no downsides to the lighter piece of gear other than the price, then I'll spend quite a bit (ie. $30/oz). I love situations where I can save weight just by spending money because it means I can be just as well equipped on the trail, yet lighter. A good example of this is down quality. I will happily spend extra money to get the best down I can, because the only downside is the price. An extra $100 to get 850 or 900 fp down means years of having a lighter and smaller packing sleeping bag with no cons aside from the initial purchase price. Conversely, some weight savings purchases continue to affect you on the trail. Often it's through lower durability, but it could also be in function (ie. carbon poles are more flimsy, some lighter tent floors are less waterproof).
Another aspect to good gear decisions is considering how your gear arsenal might change down the road. It you can't afford the best sleeping bag, should you buy a decent one now and then perhaps replace it in a year or two with a really good one? Or just wait a bit longer and buy the best now? Usually it's far more economical to just buy the best gear from the start, then it is to evolve in that direction by buying and selling many times. If you currently own a 600fp down sleeping bag, the cost per ounce to upgrade to a 700fp sleeping bag may be lower than an 800fp bag, but if you buy the 700fp bag you will either have to live with those ounces for years or upgrade again to the better bag and that will cost you even more than if you just bought the good one from the start. The bottom line for me is that I buy the best (within reason) and if I can't afford it then I buy fewer pieces of gear. I'd rather replace one or two pieces of gear per year with really good ones, rather than replace a lot of gear with more okay gear. With this approach, I will eventually have a full set of very good gear, rather than an evolving set of mediocre gear.