Example reply:
Thank you for contacting us about [describe the materials; comment on their research value if appropriate]. Unfortunately we are not allowed to provide monetary appraisals.
[If this is just to provide a ballpark value estimate; note that this relatively easy for books but hard for unique archives/manuscripts]
You might be able to find a general estimate of the value by looking for comparable items in a rare book/manuscript sales site like https://www.abebooks.com. Some things to be aware of if you check there is that the exact edition, condition and anything like autographs or other special additions to the item can affect the value greatly, so you'll want to find a copy for sale that is as close as possible to what you have. Also, just note that the prices you see there are retail prices -- if you were to sell your items to a bookseller, you could expect to get a fraction of the retail value because of course, the booksellers need to make a profit.
A nice source that explains the general values of rare books is "Your Old Books," which is put out by the professional organization for rare book librarians. It might give you some more background information about what makes particular books rare, valuable or (on the other hand) not so valuable monetarily.
[Referrals to appraisers]
You can find a list of potential appraisers through the Antiquarian Booksellers' Association of America (ABAA) member directory.
[If this is for a potential donation to the Archives, include this information]
For tax purposes, the IRS sees it as a conflict of interest for us to provide appraisals or for us to arrange an appraisal on your behalf. Basically, the valuation has to come either from you directly or from an outside qualified appraiser. If you're claiming a donated value under $500, you don't have to get an official appraisal done. If you're claiming a value between $500-$5000, then you'll have to fill out IRS Form 8283 and attach it to your tax return, but you still won't necessarily need a professional appraisal. It's only when you go over $5000 in value that you have to provide an appraisal along with Form 8283. In that case, you would need to find a "qualified appraiser" to compile a report for your taxes. Appraisals can cost a fair amount of money to have done, so you'll have to weigh the cost of the appraisal against what you anticipate getting from a deduction. We recommend talking to a tax or estate professional if you have questions. Pacific's Advancement Department may be able to provide some more information as well.
[Special rule for potential donation of personal papers/art CREATED BY THE DONOR him/herself]
There are some special tax rules about taking deductions on donations of material that you created yourself. We can't offer tax advice on this (please do consult a tax professional), but here is the basic issue. Thanks to the Tax Reform Act of 1969, people cannot take deductions on the donated value of writings, artwork, etc. that they had created themselves, except for the value of the raw materials themselves like that paper or the paint they used to make them. So for example, if an author wanted to donate his own drafts of a novel to an archive, he could only deduct the cost of the paper, not the market value of the drafts. This page includes some more information about this issue. We do recommend talking to a tax or estate professional if this situation is of concern to you.