Ni siquiera con descuento, los boletos del estreno son baratos hoy.

In Spanish, when a negative word or phrase comes before the verb, you don’t use bold no. Here, bold Ni siquiera is a negative intensifier placed before bold son, so bold no isn’t needed.

• Correct: bold Ni siquiera con descuento son baratos hoy.
• Also correct with bold no if the negative comes after the verb: bold No son baratos ni siquiera con descuento.
• Don’t say: bold Ni siquiera con descuento, no son baratos… (redundant).

Yes. That version is fully correct and common. The difference is focus/flow:

• bold Ni siquiera con descuento son baratos hoy fronts the “even with a discount” part for emphasis.
• bold No son baratos ni siquiera con descuento keeps standard SVO order and adds the negative after the verb.

Yes. bold Ni con descuento… is acceptable and means the same. bold Ni siquiera adds extra emphasis (“not even”).

• Slightly less emphatic: bold Ni con descuento son baratos hoy.
• More emphatic: bold Ni siquiera con descuento son baratos hoy.

With temporary prices (especially with bold hoy), many speakers prefer bold estar:

• More idiomatic for a temporary situation: bold Ni siquiera con descuento, los boletos del estreno están baratos hoy.
• bold ser + barato tends to describe a general characteristic (“are cheap by nature”). With bold hoy, bold están usually sounds more natural.

bold Con descuento is a set phrase meaning “at a discount” in a general sense.

• Use bold con un descuento when you specify or highlight a particular discount: bold con un descuento del 20%.
• Plural: bold con descuentos = “with discounts (available).”

For event tickets, bold entradas is widely understood across Latin America. bold Boletos is very common in Mexico and some other areas.

• Colombia/Costa Rica: bold tiquete (also means “receipt” in some places).
• Avoid bold billete for event tickets in Latin America; it usually means “banknote.”
• Safe pan–Latin option for events: bold entradas.

It’s optional. A comma after a fronted adverbial/prepositional phrase is common to mark the pause:

• With comma: bold Ni siquiera con descuento, los boletos…
• Without comma: bold Ni siquiera con descuento los boletos…
  Both are correct. If you insert bold ni siquiera con descuento mid-sentence, set it off with commas: bold Los boletos, ni siquiera con descuento, son…

bold Hoy is flexible:

• bold Hoy, ni siquiera con descuento, los boletos del estreno son baratos.
• bold Ni siquiera con descuento, hoy los boletos del estreno son baratos.
• bold Ni siquiera con descuento son baratos hoy los boletos del estreno.
  Choose the position that sounds most natural and clear in your context; beginning or end is most common.

Both mean “not even,” but bold ni siquiera is the most common and neutral in modern usage.

• bold ni aun (without accent) is correct but more literary/formal/regional.
• Remember: bold aun (no accent) = “even”; bold aún (with accent) = “still/yet.” In bold ni aun, there’s no accent.

No.

• bold hoy = “today” (this day).
• bold hoy en día = “nowadays/these days” (a general time period).
  In this sentence you want bold hoy, not bold hoy en día.

bold Barato can sometimes imply “cheap” in a low-quality sense. Neutral alternatives:

• bold económico, bold accesible, bold a buen precio.
  If you want to highlight a promotion: bold en oferta, bold rebajado, bold con descuento.

Yes. Spanish allows postverbal subjects, especially after fronted negatives/adverbials:

• bold Ni siquiera con descuento son baratos los boletos del estreno hoy.
  This can sound slightly more formal/emphatic.

In negative contexts, bold siquiera often means “even/at least”:

• bold No tengo siquiera diez dólares. = “I don’t even have ten dollars.”
  In positive sentences, use bold al menos or bold por lo menos for “at least,” not bold siquiera.