Rahisi comes from the arabic word رخيص /rahis/ which means cheap. In swahilli it can also be used as cheap along with simple.
They are two different words, both coming from Arabic but with seperate roots. رخيص /rahis/ is cheap in Arabic, and رئيس /rais/ means head in Arabic. While in Swahili they look similar, due to sounds in Arabic that were changed when transfered to Swahili (like خ) if you look at the Arabic spelling you'll see these two words are spelled (and pronounced) very differently in Arabic. So they have no lingual connection, despite similarities in Swahilli :)
Rais or raisi, the second of which sounds a lot like rahisi, means president.