There are two issues here
Are expenses of a tsedaka collector/organization considered part of your ma'aser? (your question)
Is the price of the raffle ticket/dinner considered part of your ma'aser? (not directly your question but you speak of buying a raffle ticket so quite relevant)
On the first one, poskim ask if a tsedaka collector is allowed to take a commission and agree that they indeed can. The whole amount is considered ma'aser and there is no discussion of deducting his commission from your ma'aser. For instance R Avrohom Chaim Feuer writes in his book The tzedakah treasury (p. 335)
R Moshe Heinemann relates that his Rosh Yeshiva, R Aharon Kotler,
said a professional fundraiser is allowed to take up to 49% of what he
raises as his commission. As long as the majority of the money goes
to the institution, it is considered that he was raising funds for the
institution and not for himself
@Shalom similarly reports here in the name of R Yaakov Kaminetsky that
a tzedaka can have up to 49.9% of its costs as overhead, and still
count as tzedaka vis-a-vis your tithing money.
On the second issue many poskim rule you need to deduct from your ma'asser the price you would have paid for a raffle ticket if not coming from a tsedaka organization, e.g., see here from R David Sperling
In the work Emet L'Yaakov (by Rav Yaakov Kaminetzky zt"l – Yoreh Deah 249)
it says that one should deduct the amount you would pay for such a
meal from the total donation, and only consider the remaining sum as
charity. This may not necessarily be the same amount that the
organizers paid for the meal, which may have been more or less, but
the amount you would be willing to pay for such an evening. So, if you
paid $200 say for a ticket to the Gala Dinner, and for such a meal and
entertainment you would expect to pay around $50 if it wasn't a
charity event and you were to go out for such an evening, you should
consider only $150 as the charitable gift which may be paid for with
maser kesafim monies.