I think "several tens of" could suggest "at least 30" and is semantically different than "tens of".
Some data that follows might suggest counter-intuitive results regarding usage in comparison with "several dozen of". In Google searches, "Several tens of dollars" is about four times as popular as "several dozens of dollars" and "several dozen dollars" combined:
A Google NGram search (1) shows only a few "several tens of dollars" (many of which may be a translation of a Chinese idiom) and no "several dozen dollars".
On the other hand, another Google NGram search (2) shows "several dozen" being more popular than "several tens of", as expected. However, "several tens of" is much more popular in non-fiction than it is in fiction, where "several dozen" shows an opposite trend. A quick review of the books indicates that "several tens of" is popular among technical journals; this makes sense because in that case accuracy trumps prosody.
This analysis suggests that "several tens of" could be an acceptable form, especially in technical and other non-fictional writing. Also, "several tens of dollars" seems to be more popular than "several dozens of dollars", but this is only a gross analysis.
NGram 1 = several tens of dollars,several dozen dollars,several dozens of dollars
NGram 2 = several dozen of,several tens,several dozens of,several dozen,several dozens of:eng_fiction_2012,several dozen:eng_fiction_2012,several tens of,several tens of:eng_fiction_2012