I'm assuming from your post that you have a tree claim, have been presented with an estimate and are trying to decide if the cost is appropriate?
The key to assessing any type of charge is 'reasonable and customary' - therefore cost per se is not the focus i.e. 'it is what it is' within the policy terms. Always attempt to determine the scope of damages first then determine the appropriate cost (damages); subsequently, coverage will be applied to determine the loss settlement.
Most catastrophe assignments do not include coverage or settlement authority and, there has been a trend that obtaining an agreed cost (with the insured or the insured's contractor(s) of choice) is not required or even desired by the carriers (many don't even want you to provide the insured a copy of your estimate).
On non-catastrophe claims, an independent adjuster may be given limited or full coverage/settlement authority or; may be requested to attempt to obtain an agreed cost of repairs or, may simply be requested to make recommendation only to the carrier of the damages.
Notwithstanding the type of claim (cat vs. non-cat) or the coverage/settlement authority, the adjuster is always expected to provide estimated damages that are 'reasonable and customary'. It is the adjuster's responsibility to determine damages regardless of the type of assignment
If the adjuster is provided an estimate or repair receipt, it is the adjuster's responsibility to investigate and determine if the charges are appropriate. If in doubt, the simplest procedure is to obtain a 2nd or 3rd bid. The adjuster often encounters charges that seem to be outside the 'reasonable and customary' charges (higher or lower) for the damages being considered. Most often, the adjuster can satisfactorily resolve this difference with further investigation and discussion with all parties (insured, contractor, etc) and thereby determine an appropriate cost for the damages being considered.
In general, the insurance valuation language considers settlement on 'reasonable and customary' or sometimes 'usual and customary' charges - essentially the same thing.
Obviously, if the charges being considered are higher than what is 'reasonable and customary' then they must be substantiated i.e. what are the special circumstances requiring charges higher than amounts generally considered 'higher than normal'? If such additional costs cannot be substantiated then there likely will be no coverage for the additional costs. If the insured (or the contractor on behalf of the insured) is unwilling or unable to document the additional cost then, the insured would be advised it is the insured's responsibility to substantiate the damages. If the negotiations become protracted, the insurance carrier (or the adjuster on behalf of the carrier if such authority is granted the adjuster) may decide to issue payment for the 'reasonable and customary' charges (subject to policy valuation language) and advise the insured they will consider any additional documentation the insured cares to submit in support of a differing amount.
It is a common occurrence that insureds obtain estimates from contractors who are unwilling to support the scope/amount of their estimates - basically, their position is 'take it or leave it' which often results in acrimony between the insured and the adjuster (the insured believes the adjuster is being unreasonable). While in most jurisdictions, the carrier cannot dictate which contractor the insured must use, it should be explained to the insured (by the carrier or, the adjuster if granted such authority) that they may choose whomever they want to do the work but the policy only provides 'reasonable and customary' charges and that the insured would have to pay any additional (uncovered) monies out-of-pocket.
Sometimes, the charges being claimed are actually less than what is 'reasonable and customary' and the adjuster must also exercise due diligence (as with charges that are higher than 'reasonable and customary') in determining the appropriateness of the scope/damages. This scenario happens for any number of reasons but a couple prominent ones come to mind. 1) sometimes contractors low-ball their estimate (hoping to get the work) and will later supplement the claim; unexpected supplements should always be minimized - always strive to contemplate the unexpected in your initial scope (potential hidden damage, etc.) and reserve accordingly; 2) the contractor bidding does not have the necessary expertise which is reflected in the bid (i.e. lacking in scope and costs). Either scenario has potential pitfalls which will expose the carrier and the adjuster to bad-faith claims if not handled appropriately.
It goes without saying that claim files should be documented thoroughly to include the justification(s) for actions or positions taken with regard to investigation and coverage determinations. With regard to bad-faith (most generally, an adjuster does not always have to be correct in their assessment but must always exercise due diligence (within accepted loss adjusting principles) in handling, investigating, settling or, denying (full or partial) claims.
I hope the above is helpful to you. Though the above discussion just touches on the many issues, this should give you an overall view of issues with which to address your particular claim.