I pronounced it [to.pe`r] (X-SAMPA) but I didn't realize it was Japanese. Also, thats because its similar to how my name is pronounced: [zai.be`r]. However, knowing its Japanese doesn't help because neither <p> nor <pr> is valid in Japanese syllabary as far as I'm aware. So it must be from English or another language that's loaned in. You'll notice that プ is Katakana which is reserved generally for transliterating non-Japanese words, so that theory seems to hold up. That said, [to.prei] is probably the correct pronunciation.