What to include in the price?

I think it's a fair statement.  If someone were to ask me that question (How much does it cost to do astrophotography) I'd probably respond with a paired question, though:  what equipment do you already have, and how much do you want to spend?  Obviously, not everyone _has_ an SLR or a decent PC, but those that do could get started with a tracking head and a decent tripod, a setup you and I both use.

 I shoot from a reddish orange back yard most of the time, but I've found light pollution filters to be more hassle than they're worth; Siril's background extraction tool is better for me at least 90% of the time, and Siril is free.  If someone is comfortable with a little tinkering, and has a Mac or Linux box, INDI/Kstars and Siril is an unbelievable deal, but on windows, to get a comparable experience, you've got to drop some serious coin, so I think you've got to include software costs as well.

I guess I'm saying any answer I'd give would have to be qualified; I also come from an engineering background, and I try _really_ hard not to speak in absolutes.
"If you've got a recent DSLR, a decent tripod, and a decent PC, you can get into this for the cost of a tracking head and an intervalometer, but if you have to start from scratch, it's the cost of a tracker, camera, tripod, and a modern PC."  It's a fun thought experiment, if nothing else.

Matt and Andreas, I agree with both of you. There is no simple answer to give to some one at a public star party. I guess one just has to answer a question with a several questions including what one wants to take pictures of and what one already has. I think part of the problem is there is no one system that can do it all. It is not just a matter of spending more money on a better kit, but one of buying several separate kits. There seems to be a basic incompatibility between deep sky and planetary photography.

One thing I think is good is the cost of a decent low price kit has been going down in recent years.

I always recommend to grab all the stuff from within one price category. This means, if you buy a low-budget camera body, then it is okay to add a good telephoto lens instead of investing in an expensive APO refractor. If you combine this with an affordable tripod-mounted tracker, then you have a well-balanced equipment and you can achieve astonishing results with it. The same can be applied to other applications / categories.
Let me further explain based on my own experiences: I try to follow my own advice as good as I can: Some time ago I had a self-modified mirrorless cam for 150€ and a simple 6" newton for 200€ mounted on a 150€ mount (both by TAL/Siberia). To improve the image quality I looked for a coma corrector. Apart from the high-end ASA correctors there are some mid-range correctors for 250€ and finally some simple correctors for 100-120€. At the end of the day I decided to go with the low-cost corrector, because the tiny benefit of an enhanced correction in case of the other ones would have been completely eaten up by the poor tracking quality of the mount 

Gilbert Ikezaki:
The cheaper you try to go, the more knowledge you have to have

Yes, that's it!! That's exactly my purpose of trying low-cost AP: invest a lot more brain than money, it's much more exciting! And you will learn a lot.

Astrobiscuit, that is an excellent video series. Because you explained exactly what you mean by 155 GBP, I don't think that it leaves much to be misunderstood. However, I think it illustrates the problem of assigning a number to doing low cost astro-photography. You didn't explain how you fixed the sensor in the used DSLR for no cost, and I'm not sure that such skills are very common, but for those few who have the multitude of skills you displayed, you've shown it can be done with good result. Good job!

Well, to remove the filter is only a must-have when you're capturing faint hydrogen emission nebulae. The removal of the filter is one part, but the tricky thing is then to maintain the camera's infinity position. A better option for beginners is to start with a red-sensitive DSLR. Some of them are out there, e. g. the Fuji X-A1:
http://www.astrobin.com/274187/B/

I say $2000 + a DSLR.  And that, if that number is too high, put a camera and a lens on a $300-500 camera tracker.

The $2000 is for a Sirius/HEQ5 and a small inexpensive refractor, and a few bits.  My minimum starter setup with a scope.

Matt, I feel the anti-AVX prejudice also, but I've been happy with my AVX too. I agree it is not the best mount, but it is one of the cheapest. I keep looking at better mounts, but I wonder if I want to carry all that weight around. I keep asking myself if galaxies are worth the effort.