Getting the best value ticket and "coming back as late as possible" are, in some senses, incompatible aims.
The cheapest available through ticket would be the Super Off-Peak Return routed "via High Wycombe" (i.e. for use on Chiltern), costing £55.10 before Railcard discounts. This has no time restrictions on a Saturday.
The first official itinerary of the day via this route would get you to Birmingham Moor Street for 10:06, leaving Weybridge at 06:01.
Realistically you could probably make the 07:02 from Marylebone (arriving into Moor St at 09:06) if you took the 05:42 or possibly the 06:01 from Weybridge - but these wouldn't be official connections across London. Therefore, you couldn't claim any compensation if you missed the 07:02 from Marylebone.
The last official itinerary back would mean leaving Moor St at 20:13 and arriving back at Weybridge at 23:48.
If arriving earlier or leaving later is important then you'll want to travel with Avanti. That'll cost you accordingly more: an Off-Peak Return routed "Any Permitted" costs £77.90 before Railcard discounts. That would allow you to arrive as early as 08:27 on an official connection, and return as late as 21:21.
If you only wanted to travel using Avanti in one direction, you could buy the cheaper "via High Wycombe" ticket and then obtain a "change of route excess" for the relevant portion (outward / return) for half the difference.
So if you wanted to go out using Chiltern and back using Avanti, you could do so by buying the "via High Wycombe" ticket and then obtaining a change of route excess for the return portion, at a net cost of £66.50.
Of course you might be able to save by using Advance tickets (if you're happy to be tied to a specific train) or split tickets. But the above amounts are the most you should have to pay.