Should I mention reimbursements for financial expenditures when first contacting tentative speakers for a workshop? I am thinking of organizing a workshop in my field and am currently in the process of looking for potential speakers. Some of these I'll be contacting for the first time ever by email. The speakers I am thinking of targeting are a mixture of experienced academics and relatively inexperienced/first-timers, such as postdocs. 

Clearly we will cover travel and accommodation expenses for the speakers. Should I mention this up front in the first email, or can it be perceived as rude to do so? Is it usually taken for granted that invited speakers don't pay for travel and accommodation?
If you intend to cover expenses, I would suggest saying so up front.
Some invited talks cover expenses and some do not, so it's important information to include.

You've presumably picked these speakers to invite because you really want them there.  Stating in the invitation letter that you will cover expenses will let them clearly know how much you value their presence.  In particular:


Senior scientists, who often get a lot more invitations than they can accept, will know to take you seriously because you are actually offering to pay their expenses, and they are much more likely to say yes.
Young scientists, who often have had few such experiences, will likely feel flattered and important and be very likely to say yes.


Don't make a big deal out of it (that could make you look inexperienced and desperate); just note towards the bottom of the invitation that you will cover all reasonable travel expenses. That is signal enough.