I use a fair bit of sugar(s) in my all-grain brewery, it's a handy tool to help you fine tune your beer's mouthfeel and finish. Can table sugar thin a beer's body? Certainly. Like any ingredient, you have to use it in an appropriate quantity in an appropriate application.
I'll argue that table sugar is especially useful for extract brewers because it provides the brewer with the opportunity to "tighten up" a beer's finish with 5% table sugar, or nudge the beer toward a drier finish with around 10% table sugar. I understand that extract has become vastly improved since I last used it a few decades ago. Nevertheless, whenever I provide assistance to starting extract brewers and convince them to use 5-10% table sugar the feedback is always positive because it allows them some control over their beer's finish. "It tastes less homebrewy," is a common remark.
You'll want to tailor your use of table sugar to the recipe. A wheat beer grist that only uses wheat extract is a good candidate for 5%. An American Brown Ale that really leans into the C-malts would be an appropriate candidate for something closer to 10%. Also, consider the weather. During the summer you might want to use a bit more sugar to tighten up the beer's finish and make it more crushable during the summer months. I used table sugar in an Australian Sparkling Ale this summer for exactly this reason. In contrast, you might want a January beer to be a bit rounder, even a bit flabby. In that case, you might want to omit the sugar entirely. Last, your gravity plays a big role in the appropriate amount of sugar to use: 10% sugar in a 1.042 Blonde Ale is going to play very differently than 10% sugar in a 1.078 Imperial Red IPA with lots of C-malts.
Unfortunately, like all things homebrewing, it comes down to starting out small and iterating your way toward something that fits your personal tastes, but I don't think you can go wrong by starting at 5%.
Since you're concerned about the cost of malt and adjuncts such as rice and corn (and who isn't?!), you might want to consider making your own invert sugars.
Here's a great thread on making invert. It's pretty simple and it's a cheap. All you'll need is citric acid (less than ten bucks for a lifetime supply), some Ball Jars, a sauce pan/pasta pot, and an accurate thermometer. You likely have most of that stuff already. The nice thing about invert is that it doesn't just manipulate a beer's mouthfeel, it also adds pleasant flavors of its own. It's good stuff and it's cheap.
I hope you found this useful.