Learn Hardcore Norwegian: Hotellet er rimelig i helgen. - The hotel is affordable on the weekend.

Hotellet er rimelig (not rimeligt) Compare with an adjective that does take -t:
Hotellet er stort (from stor) In the plural you add -e: Hotellene er rimelige.

Why is it the definite form i helgen instead of just i helg?

Time expressions with periods like weekend, summer, Christmas, etc., often use the definite form in Norwegian:

i helgen (this/the weekend)
i sommer (this summer) Saying i en helg is usually odd unless you mean some unspecified weekend in a very general sense. For habitual meaning use the plural: i helgene or om helgene (on weekends, generally).

What is the difference between i helgen and til helgen?

i helgen = during the weekend (context decides whether it’s the past or the upcoming one).
til helgen = this coming weekend (looking forward). Examples:
Hotellet er rimelig i helgen. (cheap during that weekend)
Hotellet blir rimelig til helgen. (will be cheap this coming weekend)

Can I say på helgen?

No. Use:

i helgen for during the weekend
på lørdag/på søndag for on Saturday/on Sunday Norwegian mixes i and på in time expressions: i helgen, but på mandag (on Monday).

What nuance does rimelig have compared with billig?

rimelig = reasonably priced, fair for the price; also means reasonable/justifiable in non-price contexts.
billig = cheap; can imply low price and sometimes low quality. Both are fine for prices, but rimelig is a bit softer/politer. Synonyms you may see: prisgunstig, gunstig, ikke dyr.

If I want to talk about weekends in general, how do I change the sentence?

Use the plural definite for weekend(s):

Hotellet er rimelig i helgene. (The hotel is affordable on weekends, generally.) You can also say om helgene for habitual meaning.

Can I move the time phrase to the front?

Yes. Fronting the time phrase is natural and adds emphasis to it:

I helgen er hotellet rimelig. Word order after fronting stays Subject–Verb: the verb er still comes right after the fronted adverbial in main clauses.

How do I turn it into a yes–no question?

Invert subject and verb:

How do I say it in the past or future?

Past: Hotellet var rimelig i helgen. (was cheap over the weekend)
Future (neutral): Hotellet kommer til å være rimelig i helgen.
Future (plan/expectation): Hotellet blir rimelig i helgen. or Hotellet skal være rimelig i helgen.

What are the plural and adjective agreements if there are several hotels?

Definite plural subject: hotellene
Adjective in plural takes -e:
- Hotellene er rimelige i helgen. For a generic statement with weekends in general:
- Hotellene er rimelige i helgene.

How is the sentence pronounced?

Approximate Urban East Norwegian:

Hotellet [hʊˈtɛlːe] (the final -t in -et is often silent)
er [æɾ]
rimelig [ˈriːməli] (stress on the first syllable)
i [i]
helgen [ˈhɛljən] (the lg becomes a kind of lj sound) Put together: [hʊˈtɛlːe æɾ ˈriːməli i ˈhɛljən]. Tip: keep the double ll in hotellet long, and make i a clear [i].

Why do some people say i helga instead of i helgen?

Helg can be masculine or feminine in Bokmål:

Masculine: en helg – helgen
Feminine: ei helg – helga Both are correct in Bokmål; usage varies by region and personal preference. In Nynorsk you’ll typically see helga. Similarly, rimelig is Bokmål; the Nynorsk form is rimeleg.

Does rimelig ever mean something other than price-related?

Yes. It also means reasonable/justifiable:

Det er rimelig å anta at… (It is reasonable to assume that…) And in informal speech it can function like an adverb meaning pretty/fairly:
Det er rimelig kaldt. (It’s pretty cold.) In this sentence with a hotel, the price meaning is the one understood.