Make UnitPrice read Only using triggers

Question

Currently I am trying to block certain profiles from changing UnitPrice within Quote. I used validation rules under customize> QuoteLineItem and worked perfectly fine. However those validation rules only work when the product has been already added. The Issue I have right now; is when the user adds a product inside quote by clicking on "Add Line Item" they are prompted to a page where they can enter quantity. There is also a UnitPrice there as well. They are still allowed to change the price. I am not sure how i can access that page and how I can disable that field within the page. I thought the below trigger could do it, but I am not sure how to add certain profile or even where do i add that trigger to for this page.

Check if the value of "UnitPrice" has changed like

if(trigger.new[0].UnitPrice != trigger.old[0]).UnitPrice)
{
trigger.new[0].addError('You can not change the value of SalesPrice');
}

currently this is what I have as validation rule that stops user from changing price after the lineItem has been added

OR(AND(ISCHANGED(UnitPrice),$UserRole.Id  = "00E..."),AND(ISCHANGED(UnitPrice),$UserRole.Id  = "00E..."))

Updated: the new validation rule using my own profile ID as a test case

OR(
    AND(ISNEW(), NOT(ISNULL(UnitPrice)),
    AND(NOT(ISNEW()), ISCHANGED(UnitPrice)),$UserRole.Id = "00EG00....")
)

Answer 1

Stick with your Validation Rule. You just need to add a clause for ISNEW.

OR(
    AND(ISNEW(), NOT(ISNULL(UnitPrice)),
    ISCHANGED(UnitPrice)
)

If you want to add a UserRole blacklist to the mix, I recommend the following:

AND(
    1 = CASE($UserRole.DeveloperName,
        "Admin", 1,
        "OtherBlacklistedRole, 1,
        "etc...", 1,
        0
    ),
    OR(
        AND(ISNEW(), NOT(ISNULL(UnitPrice)),
        ISCHANGED(UnitPrice)
    )
)

Note that both formulas should work equally well for the insert and update case. If you really insist on keeping the insert and update rules separate, they would be:

Insert Rule

AND(
    1 = CASE($UserRole.DeveloperName,
        "Admin", 1,
        "OtherBlacklistedRole, 1,
        "etc...", 1,
        0
    ),
    ISNEW(),
    NOT(ISNULL(UnitPrice))
)

Update Rule

AND(
    1 = CASE($UserRole.DeveloperName,
        "Admin", 1,
        "OtherBlacklistedRole, 1,
        "etc...", 1,
        0
    ),
    ISCHANGED(UnitPrice)
)