So I went by the local installer today to get an idea of their first hand experiences with these options. They do lots of BMW's and incidentally there was an M240i in the lot. Kid was getting a big sub box for the trunk. Space or usability was not a concern for him
I'll paraphrase our chat but here's the gyst - there are effectively 4 ways to do this (we're talking about the non HK system here because that car has a different upgrade path).
I'll invent the levels and call them: budget, standard, baller, pro/competition using online pricing. You should expect to pay more over the counter but get warranty/returnability, and less if you go the ebay/craigslist route.
Each level has only a minor increase from the one before. So from budget to standard is a minor bump in quality, and from standard to standard plus is a minor bump again, and from standard plus to baller is a minor bump.
Only when you skip levels is there a considerable jump. The nice part is there is only a couple hundred bucks between levels as shown here:
"budget" is to not touch the amps but simply replace the 4 speakers with identical coax with Built in BMW mounts and harness (don't buy speakers that need extra pieces as any money you save you end up spending more in and at this price point it isn't worth it - unless you have a shop install and they don't charge extra for the parts)...and you're done. There is no upgrade path from here and it would require replacing the speakers again with something else should you go that route to hear better audio. You could amp/DSP them for an improvement, but you would be better off starting with better speakers there in the first place. Cost under $300 before labor.
I'm not sure if anyone here looking at the audio upgrade section would be truly happy with stopping here, maybe the guys who are more into performance mods where adding audio means added weight, but I thought I would add it in.
"standard" is what you typically see here in the forums. high end coaxials or low end components in the front (they like the OEM sail panel method vs just bolting the tweet to the blank panel and didn't feel there was a serious difference between low end comps and high end coaxials due to the mounting limitations). Leave the rear alone. Dampen the doors. Cost is under $450 before labor. Again you need to only pick from speakers that have the BMW mounts and harness to stick to the price point (unless you have a shop where this is just included in the install labor). But there is an upgrade path this time.
"standard plus" gives you an amp/dsp. The local guys prefer something like the Audiotech Fischer to the JL amp. Obviously when you take out the carpet to change the amps you dampen the trunk. Figure $600 in addition to the standard upgrade not including labor.
This is effectively the end of where most people would be more than happy.
So all said and done you are at $1000-1500 in parts before labor to do this at an acceptable level. Figure $1k for something that is BMW friendly and $1500 for something superior that needs custom mounts/adapters/harness. If the speaker mounts up with no rings you save some cash.
No sub upgrade is needed. In addition, they emphatically recommend NOT getting high end speakers at any stage unless you know up front that you are installing a decent amp. They feel without an amp or DSP you are wasting your money on high end speakers.
Also - at the same price point - they say that the sonic differences in getting a speaker with the BMW mounts built in or having to retrofit a 'fancier' speaker that needs custom mounts are more personal preference and timber as opposed to actual superiority. Again, remember keeping the price the same - and at this level they are close enough for jazz. So unless you must have the 'fancier' brand with the 4 hole - or unless they are significantly cheaper - just get the ones designed to fit from the get go. The money spent on the mounts and harness would be better spent towards an amp/DSP.
I should stop here but promised 2 more levels. From here you can have some fun if you have money burning a hole in your pocket.
What I call "baller" is high end components all around, amp/dsp, dampening the doors and trunk, and new subs. Preferably all by the same company for the ability to name drop if that's important to you. The cost here is about $2500. The difference here between this and standard plus is basically just using the highest end components in front with the mid-bass subs under the seat (the latter of which are the bulk of the cost of the upgrade).
You can do a trunk sub and amp too for another grand on top...call it 'extra baller.'
Above that is just custom everything. No real budget here. But we've long gone past sanity.
Seth