Learn Swedish: Priset är lägst idag. - The price is lowest today.

Lägst means "lowest", and its base form is låg (low). This is an adjective that changes its vowel in the comparative and superlative forms. The pattern is låg (low), lägre (lower), lägst (lowest). Notice how the å changes to an ä.

Why don't we say lägsta here?

When a superlative adjective comes directly after a verb like är (is) and stands alone describing the subject, it takes the bare -st ending. You only use the -a ending (lägsta) when the adjective is grouped together with a definite noun, like in det lägsta priset (the lowest price).

Why does "the price" end in -et?

The word for price is ett pris. Because it is an ett-word, you add -et to the end to make it definite ("the price"), becoming priset.

I sometimes see idag written as two words (i dag). Which is correct?

Both idag and i dag are completely correct and mean "today". You will see Swedish speakers use both interchangeably, though idag as a single word is very common in everyday writing.