Of course, you should ask but several factors can give you an idea of relative prices.
As a rule, drinks may be priced by 'moves'. A gin and tonic is a two-move drink so it will normally be cheaper than a fancy martini. A sea breeze is a three-move drink but, since two of the ingredients are fruit juice, it may be only slightly more expensive than a gin and tonic.
A gin and tonic made with 'well' gin (say Gordon's) will be cheaper than one made with Bombay Sapphire.
You also have to be careful with wine. I once ordered a glass that turned out to be more expensive that Mr. Thipu's Maker's Mark. From that point on, it was house wine only.
Still, it's always best to ask. No one will think any the less of you for it.