Should I mention reimbursements for financial expenditures when first contacting tentative speakers for a workshop?
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.