>11 CFR 110.20 - Prohibition on contributions, donations, expenditures, independent expenditures, and disbursements by foreign nationals (52 U.S.C. 30121, 36 U.S.C. 510).
This is the relevant law.
>(b)Contributions and donations by foreign nationals in connection with elections. A foreign national shall not, directly or indirectly, make a contribution or a donation of money or other thing of value...
"Other thing of value" here is opposition research, which has real monetary value (i.e. it costs something, hence the distinction from "money").
>(g) ... No person shall knowingly solicit, accept, or receive from a foreign national any contribution or donation prohibited by paragraphs (b) through (d)
You do not need to accept or receive a contribution, only solicit one. So what is a solicitation?
>A solicitation is an oral or written communication that, construed as reasonably understood in the context in which it is made, contains a clear message asking, requesting, or recommending that another person make a contribution, donation, transfer of funds, or otherwise provide anything of value.
Asking, requesting ... anything of value. So did Don Jr. "solicit"? Here are some snippets from his emails:
>Goldstone:This is obviously very high level and sensitive information but is part of Russia and its government’s support for Mr Trump
>Don Jr: Seems we have some time and if it’s what you say I love it ... Could we do a call first thing next week ... ?
The solicitation here is clear -- an offer of "sensitive information" that is "part of Russia['s] support for Mr Trump" is made, and Don Jr. is quick to set up a meeting to receive the supposed information.
Problem with these argument:
>Does opposition research, or information in general, have monetary value according to campaign laws?
That isn't clear, and as far as I can tell has never been tested in court. You can't put an exact dollar figure on information.