My advice to you is to go there and arrange the tours on the ground rather than pre-arrange from abroad where you get charged exorbitant prices.

There will be PLENTY of people selling tours to you, be it from your accommodation or just stalls along the roads. Given Bali is so touristy, most of them are standardized and eventually run by the same companies.

Based on my experience when I was there earlier in March this year, these are the activities I booked via the guesthouse in Ubud I was based in and the prices I was charged :-

Cycling Tour with Arek Arek Bali
- standardized cycling tour where they bring you up the mountain first for breakfast (and stopping by an agroculture plantation beforehand to drink Kopi Luwak) and then cycling most or less downhill all the way after that (which makes things much more enjoyable)
- price charged IDR350,000 (USD27) - same as on brochure given
- I was the only one on this tour (like a private tour) so I couldn't compare prices

Rafting Tour
- half day enjoyable rafting tour down the rapids
- price charged IDR550,000 (USD42) - brochure given showed price of USD50
- however another guy doing the same trip said he booked it from some roadside stall for IDR350,000 (USD27) but he subsequently heard of others getting it for IDR250,000(!!!)

Mount Batur Sunrise Hike
- early morning 3am hike up Mt Batur for a spectacular sunrise view
- price charged IDR400,000 (USD30)
- found out the other 3 girls who joined my group paid a total of IDR700,000 (USD54) for 3 of them while an Indonesian guy in another group paid IDR370,000 (USD28) in total for the 3 in his group!

So in short, all the prices are highly negotiable and much much cheaper than what you can see online. Best bet is to go there and just negotiate with them. From my experience, the stalls along the main touristy places seem to give a lot more discounts compared to say booking through your guesthouse given there is "more competition on the street"

Hope the above helps and you will enjoy your time in Bali!