Interesting idea. Certainly worth experimenting, and I’d love to hear how you get on. No reason to follow the crowd.
In terms of ££ – an LDR costs about 20p, and an IR sensor (tx/rx pair) costs about 25p. By the time you’ve added your own LED transmitter there may not be much in it. [Prices based on multi-packs from 4tronix – other suppliers are available.]