Search the board for 'sleeve' or 'sleevecase'. I don't know how much is too much, since protecting a US$2000 tool with inestimable data with a small percentage of cost for protection is almost a requirement, but I spent $100 on an
Acme Made Slim, and they're highly recommended. Here are the advantages:
1) quilted, soft interior
2) integrated plastic on the sides and bottom for extra protection; survives many plane flights
3) side zip can store power adapter and mouse
4) top loading zip, so you don't have to remove the case from a shoulder bag
5) leather-lined handles
6) comes with shoulder strap
Disadvantages:
1) expensive
2) side zip is not particularly roomy
*There is no side zip for the Medium 15". It's a pocket. (Just got one.)
Other manufacturers and models are STM, RadTech, Waterfield Bags, Crumpler, ZeroShock, and various others like Targus, eBags and the like.