Yellow Topaz and Yellow Sapphire are both gemstones in their own right and I don't think either is a subtitute for the other.
Prices vary considerably depending on quality and size. In general, good Yellow Topaz under 3cts can range between US$15-$55/ct. Good Yellow Sapphire under 3 cts will go for about US$40-$90/ct.
Personally, I think Yellow Sapphire makes for a better jewelry gemstone, because it is more durable than Topaz. Sapphire is 9 on the Moh's Scale of hardness; Topaz is 8. Topaz also has an unfortunate physical property - basal cleavage. If hit at a vulnerable spot, Topaz will break easily.
As for whether it is good to wear with a pearl ring depends largely on your pearl, the design of the ring and on your sense of aesthetics. I don't see why it can't be used with pearl really...
Hope that helped!
I LOVE gemstones and am a gemologist