Price is kind of a meaningless construct when it comes to luxury items, but I get your point. Normally the SLP shirts I buy are fairly distinctive, meaning I can't really get them from anyone else, like the Hawaiian shirts @needhelp123 mentioned, or the silk shirts. For the more standard shirts, I think you have some options, though fit will be an issue. It all depends on what you value. While a shirt from Brioni or Charvet will undoubtedly be of a higher quality, and for about the same price, it's not the kind of shirt you'd wear with jeans. It just looks strange to me. I think the quality of SLP shirts is about on par with similar luxury houses (Lanvin, etc.), though Saint Laurent still occasionally charges more for their items. It's just easier for me to grab a Saint Laurent shirt and not have to try on a dozen alternatives.
It is overpriced but ultimately it comes down to quality and cut. Their permanent collection white shirts are perfect for me - shoulders, chest, sleeve length, overall fit. The cotton seems of a good quality though I am no expert here. The boots are an easy buy because there are no bells and whistles, the proportions are just perfect. Jeans are hit and miss depending on the season and cut.
Gonna wean off SLP soon. Found a cheaper alternative for safe classics - AMI by Alexandre Mattiussi :-)