。
It came out cheaper than I thought.
Literal
Thought-[than-より] cheaply came-out.
安くあがる ('to come out cheaply, to come in under budget') uses あがる ('to rise, to be completed') in its idiomatic sense of 'to end up as, to come out to' — specifically for costs and expenses. 思ったより ('than I thought') compares the actual result to the speaker's expectation. A common everyday expression of pleasant surprise.