I think that the ability to teleport immediately next to an altar and a lectern is worth the 3-5m that it costs to get to level 75. It'll save you hours if you plan on getting Turmoil.
Hehe, back when free trade and wilderness were not removed yet I allways kept my gilded altar open for players while I alched my magic to 99. All donations pretty much covered the cost of alches.
Its pretty much worth it. But if you can get the altar at 70 construction. Its possible if you have finished a quest called "Eyes of glouriphie". You will get a crystal saw as a reward which boosts 3 levels of construction. You can also make a tea at your own house which boosts +2/+3 construction =]. I got one on my turmoil pure and is botting happily right now XD