Bike rental there not cheap. Depends upon spec but count on around 30euro per day for a decent carbon road bike and 40euro plus for a better one.
Supply and demand plays a part, if you go peak cycling time which is April/May and Sept/Oct the rental prices get hiked.
However, if going for less than a week it's better to rent than take own.
If you do go Sa Colobra is THE climb but it's a tough one, well I thought so! The most iconic is Cap de Formentor lighthouse, absolutely awesome scenery and an absolute must ride. Be ware though at peak times it gets very busy but so worth it.
Although €30 per day is actually quite cheap for bike rental, it is more expensive hiring in France and Italy
Sa Colabra is defo iconic for its sweeping hairpins and views, although its an odd one as its a dead end so you actually ride down it before you climb back up. In peak times you share the roads with loads of tourist buses though so go early morning
The ride from Port Pollenca to Formentor Lighthouse is definitely IMHO the prettiest ride / best views etc. absolutely stunning.
The toughest climb is from Soller up and over the highest point of the Island Puig Major, particularly as you've almost certainly had to climb over the Col de Soller just to get there!
The good thing about any climb in Mallorca is there is nothing must above ~7% gradient and the surfaces are all pretty smooth.
Port Pollenca / Alcudia is a good base if you want the option of flat or hills. My mate has a place near Peguera, so I often ride from there, but that always means its a hills ride to a greater or lesser degree.
If you search on here you'll find a ride / holiday report from someone who stayed in one location but used the local train network to get some variety into his rides and see more of the island.
https://www.cyclechat.net/threads/the-water-in-majorca.295618/page-3