With clients/prospects
Drinking- Do NOT get drunk.
Food- Do not order the most expensive item on the menu and in the converse, do not order the cheapest item on the menu.
As I am typically dining with prospects/clients, I pick up the tab. The client value would determine the cost of the restaurant. (i.e. clients or prospects with a potential value of a few hundred thousand in profit or more would end up in a high end restaurant. Conversely, a client/prospect who would be valued in the sub 10k profit range would find an appropriate casual restaurant (sub 100 meal for 2-4).


With colleagues:
Drinking- Don't drink excessively.
Food- Order what you want. However, do not order the most expensive item on the menu- especially when dining with the boss. However, this rule goes out the window when dining at a cheap joint (i.e. fast food)
Typically the most senior person picks up the tab.

Please keep in mind the occasion and reasoning for the meeting. That would ultimately provide the largest guideline on how much to spend. I've had a single dinner meeting expensed with a client (2 persons), my CEO, COO, and myself that cost just under $7,000 (meal, wine, and gratuity included). However, this client was responsible for almost 30% of the company profits (close to 2MM). The CFO didn't even bat an eye at my expense sheet.