Here's the shape of the spend on a 300k wrap, with the caveat that every number below varies by state and by who you hire.
Document drafting is the big one. A wrap note plus deed of trust or mortgage, written by an attorney who has done wraps before, is generally quoted in the low four figures rather than the hundreds, because the note has to coordinate with a senior loan that stays in place. The 500 dollar figure you saw is a generic note form. It won't address what happens if the seller stops paying the underlying, and that's the clause the whole deal turns on.
Closing costs run through a title company or closing attorney depending on your state. Title insurance is usually available, and the title work will show the senior lien, so the policy and the closing statement will both reflect that the old loan is still there. Ask the title company directly whether they'll close a wrap before you get far, since some simply won't.
Monthly servicing through a third-party company commonly runs in the range of low tens of dollars per loan per month, sometimes with a setup fee. If they're also disbursing the underlying payment, expect that to be priced higher or handled as a separate escrow instruction.
One thing your list is missing: recording. In most states the wrap security instrument gets recorded, which puts a document referencing the transfer into the public record where the senior lender's monitoring can see it. Some states also charge transfer tax on the sale. Both of those are worth pricing and understanding before you talk to the seller, and the recording question is one for your closing attorney.
