Do you have this sentence from a book or write it yourself?
Apart from oishii meaning tasty and not how much (which would be ikura) I dont exactly get the right touch to this sentence
I would say
(which I dont thing is 100% right but ...)
Kono momo yô ni kore momo wa hô ga oishii desu ka?
Which would mean compared to this peach is that peach more tasty?
or
Kono momo to kore momo, dore ga motto oishii desu ka
this and and that peach, which is more tasty (done know if motto is the right word)
for how much is this and that I would use
Kono momo to kore momo wa ikura desu ga
This and that peach, how much are they
but I think this would mean how much are they together
For seperating I would use
Kono momo wa ikura desu ka, kore momo wa ikura desu ka
this peach cost how much, that peach is how much
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to can mean
and, with and also for "it is" at things like to iu
examples
Xsan to Ysan wa Gakusei desu.
X and Y are Students
Xsan to Ysan isshou ni gakko e ikimasu
Xsan goes with Ysan to (the) school
Kono Gakko wa Z to iu (dont know ending')
This School is named Z
to is also used with omou (to think)
Ano onna no ko wa kirei to omou imasu
I think this girl over there is pretty
ano i just know as are no, meaning This Subjekt/Object over there (not near speaker or partner) see above example
with ga I too have my dear problems but this is what I believe to know
You can use it as "but"
Kono kisa wa ookii desu ga, hontô ni takai desu.
This Café is big but expensive
it also is the direct object Marke
Depado ni saifu ga (stolen))
At/In the Departmentstor my purse was stolen
I rember this with What was stolen the purse so the ga is connecting the verb with the object
With the other usages, like in passive and kausative, I am not 100% sure yet
but I think this things I mention should be correct