After geta, Icelandic usually uses the verb form called the supine (Icelandic: sagnbót), not the plain infinitive.
The verb is að muna = to remember, but after geta it appears as munað:
This is very common with geta:
So munað is the expected form here.
In a normal Icelandic main clause, the finite verb usually comes early in the sentence, and ekki normally follows that finite verb.
So:
Ég get ekki munað verðið.
is the normal order.
A useful pattern is:
Subject + finite verb + ekki + rest
For example:
Verðið is the definite form of verð, which means price.
The ending -ið is the attached definite article for a neuter singular noun.
So here:
munað verðið = remember the price
Because the sentence means the price, not just price in a general sense.
Icelandic usually adds the definite article directly onto the noun:
English uses a separate word the, but Icelandic often puts that meaning at the end of the noun.
Because the expression is að giska á = to guess / estimate.
This is a fixed verb + preposition combination, so it is best learned as a whole unit:
giska á e-ð = guess something / estimate something
So:
ég giska á tuttugu þúsund
means
I guess twenty thousand / I estimate twenty thousand
The preposition á does not translate literally as on here. It is just part of the expression.
Here á takes the accusative.
With a phrase like tuttugu þúsund, the case is not very visible on the surface, but grammatically the number phrase is functioning as the object of giska á.
So the important thing for a learner is:
giska á + accusative
It means twenty thousand.
So:
tuttugu þúsund = 20,000
In this sentence, it most likely means twenty thousand krónur, but the currency is omitted because it is understood from the context.
Icelandic often leaves out words that are obvious from context.
If people are already talking about a price, then tuttugu þúsund naturally means something like twenty thousand krónur.
So the sentence does not need to spell out the currency every time.
It can be either, depending on context.
In everyday speech, especially with numbers, giska á often means make an approximate estimate:
So in this sentence it has the sense of:
I can’t remember the price, but I’d guess about twenty thousand.