I've only had my water pump go, and that was $200 at a local indie shop. If I proactively did it, it would be $60 in parts from the USA and a relatively easy DIY install.
BMW repairs are pretty cheap if you can DIY them and get parts from the USA online sellers. Local dealers are 2-4 times more expensive.
Forums like maxbimmer, e46fanatics, bimmerpost, etc. are great resources for troubleshooting and DIY tutorials.
Expansion tank is about $70 and DIY'able.
Subframe depends on the damage and type of repair. Could be hundreds if you can weld yourself. Probably $1000 at an indie shop, and $2-3000 at a BMW stealership. However, if you are persistent enough you can get them to fix it for free.
Other things I've done are the thermostat (easy), cracked intake tube (easy), coupe door trim gluing (easy).