彼女へのあげたプレゼントはご千円もした。The bolded part is も and した right?
Why is the particle も used there? While も rarely poses a problem in understanding a sentence, I often feel like I don't really understand why it is there :(
「も」 has far more meanings than people tend to think.
This 「も」 expresses a surprise or deep impression.
"(It) cost me a whopping 5,000 yen."
It is saying that it was way more expensive than expected.
See definition #5 in デジタル大辞泉.
If you are not surprised by the price, you could just state the price as:
「～～は5千円した」 without a 「も」