This was the original contract written by Michelangelo's agent for the commission of David. Honestly, design/art for hire contracts have not changed too much in the subsequent years. Maybe append something about "deployment costs" and you could reuse this now.
Die VII mensis augusti, 1498
Be it known and manifest to all who shall read this present writing that the Most Reverend Cardinal did San Dionisio has agreed that Mestro Michelangelo, statuary of Florence, that the said Maestro shall at his own proper costs make a Pieta of marble; that is to say, a draped figure of the Virgin Mary with the dead Christ in her arms, the figures being life-size, for the sum of four hundred and fifty gold ducats in papal gold (in or papali ), to be finished within the term of one year from the beginning of the work. And the Most Reverend Cardinal promises to pay the money in the manner following: that is to say, imprimis, he promises to pay the sum of one hundred and fifty gold ducats in papal gold before ever the work shall be begun, and thereafter while the work is in progress he promises to pay to the aforesaid Michelangelo one hundred ducats of the same value every four months, in such wise that the whole of the said sum of four hundred and fifty gold ducats in papal gold shall be paid within a twelvemonth, provided that the work shall be finished within that period: and if it shall be finished before the stipulated term his Most Reverend Lordship shall be called upon to pay the whole sum outstanding.
And I, Iacopo Gallo, do promise the Most Reverend Monsignore, that the said Michelangelo will complete the said work, within one year, and that it shall be more beautiful than any work in marble to be seen in Rome today, and such that no master of our own time shall be able to produce better. And I do promise the aforesaid Michelangelo, on the other hand, that the most Reverend Cardinal will observe the conditions of payment as herein set forth in writing. And in token of good faith I, Iacopo Gallo, have drawn up the present agreement with my own hand the year, month and day aforesaid. Furthermore, be it understood that all previous agreements between the parties drawn up by my hand, or rather, by the hand of the aforesaid Michelangelo, are by this present declared null and void, and only this present agreement shall have effect.
The said most Reverend Cardinal gave to me, Iacopo Gallo, one hundred gold ducats of the chamber in gold (ducati d'oro in oro di Camera ) some time ago, and on the aforesaid day as above set forth I received from him a further sum of fifty gold ducats in papal gold.
Ita est IOANNES, CARDINALIS S. DYONISII
Idem Iacobus Gallus, manu proprio.