This is a hard question to answer, because although it may seem to be a simple matter, the cost will depend on the level of expertise held by the attorney you hire. Most likely you are talking about the cost of work in the hundreds of dollars, rather than the thousands.
You may want to see if your local Bar Association can give advice on what the cost would be and see if they can refer you to an attorney who has the requisite experience to hand the transaction for you.
This answer does not constitute legal advice and does not and is not intended to create an attorney-client relationship. The law may vary depending on the state in which you reside. It is intended only to give some direction in which to seek assistance.
I agree with the previous response, and would add that the cost of drafting the agreement can vary depending on how the other party reacts to the agreement. Remember that in most cases, the purchase agreement drafted by your attorney will be an offer, and the other side may be having an attorney or a realtor review the document. If the other party asks for changes, you may be looking at having your attorney either review a counteroffer, or redrafting the agreement. That said, I do agree that a purchase agreement alone (assuming that it is a simple residential transaction) will probably cost hundreds rather than thousands of dollars. However, you should know that if the attorney was doing everything that he or she should (reviewing legal descriptions and other due dilligence) that the cost may creep over 1k.