Last time

Informally, yes. It's rather casual.

Thanks, Setanta.

From my perspective, "last time" refers to one instance. "Formerly" can be used in place of "previously."

Thanks, Roberta.

As 'last time' refers to one instance, shouldn't it be 'the last time' instead?

To have the sentence make sense, it should read:

The last time I was here, things were cheaper.

Yes, the "the" is needed with last time, but without a modifier (I was here), the sentence sounds incomplete.

The last time things were cheaper, something else.