Drumul la mare este mai scump vara.

Because drumul is the definite form of drum.

In Romanian, the definite article is usually attached to the end of the noun:

• drum = road / trip / way
• drumul = the road / the trip / the way

So drumul la mare means the trip to the seaside or the road to the sea, depending on context.

It can mean either, but in everyday Romanian this often refers to the trip/journey to the seaside.

A native English speaker might expect something very literal, but Romanian often uses la mare as a common expression meaning to the seaside or at the seaside.

So in this sentence, the most natural idea is probably:

• Going to the seaside is more expensive in summer or
• The trip to the seaside is more expensive in summer

Here mare is being used in a general, idiomatic way: la mare = to the seaside / at the seaside.

After prepositions, Romanian often uses the noun without the definite article when speaking generally:

• la mare = to the seaside
• la munte = to the mountains
• la școală = at/to school

If you said la marea..., that would usually introduce a more specific noun phrase, such as la Marea Neagră (at the Black Sea).

Mai scump means more expensive.

Romanian usually forms the comparative with:

• mai
  • adjective

Examples:

• scump = expensive
• mai scump = more expensive
• ieftin = cheap
• mai ieftin = cheaper

So:

• este mai scump = is more expensive

If you want the most expensive, Romanian uses:

• cel mai scump = the most expensive

Because scump agrees with drumul, which is masculine singular.

Romanian adjectives agree with the noun in gender and number:

• masculine singular: scump
• feminine singular: scumpă
• masculine plural: scumpi
• feminine/neuter plural: scumpe

Since drumul is masculine singular, the correct form is:

• drumul ... este mai scump

Even though scump comes after este, it still agrees with the noun.

In this sentence, Romanian normally keeps the verb a fi (to be).

So:

• Drumul la mare este mai scump vara. = correct
• Drumul la mare e mai scump vara. = also correct, more conversational

But leaving out the verb completely would not be natural here.

So Romanian behaves more like English in this kind of sentence:

• subject + to be
  • adjective

Because Romanian often uses season words adverbially with no preposition:

• vara = in summer / during the summer
• iarna = in winter
• primăvara = in spring
• toamna = in autumn

So vara here functions as a time expression.

English usually needs in:

• in summer

Romanian often does not:

• vara

You can also hear:

• în vară

But that often sounds a bit more specific depending on context, while vara is a very common general expression.

The basic order is:

• Drumul la mare = subject
• este = verb
• mai scump = predicate adjective
• vara = time expression

So the structure is roughly:

Subject + verb + adjective/comparison + time

Romanian word order is fairly flexible, so you could also say:

• Vara, drumul la mare este mai scump.

That puts more emphasis on summer, but the meaning stays the same.