konseoteu pyoga jinjja bissaneyo.

The ending -네요 combines:

• -네: indicates the speaker’s reaction or realization (often surprise or noticing something new)
• -요: makes it polite.
  So 비싸네요 implies “Wow, they’re expensive!” or “I notice that they’re expensive,” in a polite way.

진짜 is a casual intensifier meaning “really” or “so.”

• 정말 is more neutral/formal, while 진짜 is colloquial and common in spoken Korean.
  Both mean “really,” but 진짜 sounds more conversational.

Yes, you can swap them, but the tone changes:

• 정말 비싸네요: neutral/polite
• 매우 비싸네요: more formal or written style (“very expensive”)
• 진짜 비싸네요: casual, spoken style (“so expensive!”)

Using -가 here:

• Marks 표 clearly as the subject whose property is “being expensive.”
• Makes it a straightforward statement: “The tickets are expensive.”
  If you said 표는 비싸네요, you’d be topically contrasting or emphasizing “as for tickets, they’re expensive,” which can imply you’re comparing to something else.

It’s polite but not overly formal.

• -요 ending = polite speech (used with strangers, colleagues, etc.)
• Not as casual as dropping 요, and not as formal as honorific or literary forms.

Yes. Adding 많이 (“a lot”) is grammatically fine:

• 콘서트 표가 진짜 많이 비싸네요 = “Concert tickets are really very expensive.”
  Just note you’re stacking intensifiers: 진짜 많이 비싸다.

You can front 진짜 for extra emphasis:

• 진짜 콘서트 표가 비싸네요 still means the same.
  Korean word order is flexible, especially with adverbs and intensifiers, but the default (콘서트 표가 진짜 비싸네요) is most natural.