What are you using as a router, a Dremel Tool? 1/2 or even 3/4 inch ply isn't going to cut it as a router table top with a full sized router attached. I suppose you could attach supports under the table to possibly prevent sag, but I still wouldn't trust it to stay level from one day to the next. Using 2 sheets of 3/4 inch MDF glued together topped with Melamine is the standard. You could build a nice table top for around $50.00 which is probably $150.00 less than what you propose as a router table top. The benefits of a thick table top and a phenolic insert plate far outweigh using a 1/2 or 3/4 inch ply top fitted with a ply insert and will cost less over the same time span.
Spend a few bucks for an insert plate too. They can be had for less than $25.00 and along with that small investment you get peace of mind that your router will stay level if installed properly.
I absolutely do not take short cuts with my routers or accessories. As an example I just bought two base plates, one round and one offset, for a DeWalt trim router that cost $80.00 the pair. I have a $100.00 1/4 inch aluminum insert plate in my table that will out last me plus stay level, so it was a wise investment in that respect. The point I am trying to make is, it only takes a few more cents to go first class thereby preventing the hassle of re-doing something a few days or weeks down the road.
When something is advertised as being foolproof there is always a better class of fool that comes along to prove them wrong.