It's always an interesting decision as to whether to take on a "low/no budget" job, especially for those coming new to the film and TV industry. I think the most important thing to remember - whether you are 'working' for old friends or new ones - is to remember your worth.
When being asked to work for free - (which let's be blunt - is in fact illegal, as every job has to pay the national minimum wage) - it's worthwhile remembering that you're not just working for "no money", but you are in fact supporting the production by providing them with your skills and experience. So you are funding the production.
When starting out, this equation may seem to be simple enough, as you may feel that a lack of experience means that you can offer a production very little; but this is rarely the case - once again, value your worth - what are you bringing to that production: Yourself? Your (rather expensive) education? Your car (as you've probably only been offered petrol expenses) ? Your Mobile phone? Your kit ? (And are they going to insure it?)
But what is on the other side of that equation: what are you going to get from the production? It is this answer that essentially tells you whether to take the job or not. Even when being paid, there may be certain things that attract you to / or away from a job (the script, the crew, the technical requirements, the proximity of the shoot etc) meaning that you'll work for less / more money. And, (in my opinion at least), the same thinking can apply to a low budget production - try and quantify what you are going to get from it: working with a new piece of kit / working with an HOD who is already established in the industry / wanting to try a new technical set up (new radio microphones / new hard disk recorder / new firmware upgrade) / testing out a new crew relationship.
But - and it's a fairly big but - be very aware that experience that you may build up on low budget productions will be (prepare for a sweeping statement here) "low budget" experience; and not necessarily be transferrable to fully paid work. It's very much dependent upon the productions involved, but, for example, with a "no-wages" production there is no financial penalty of working over-time; so it's often taken for granted that crew will work beyond their agreed hours. Which is the ultimate irony / insult, as now you're giving your time at an overtime rate for free.
I can remember when being asked to work for nothing on a short film many years ago, I politely replied I would happily work for free if the director would come and do an equivalent hours manual labouring for me (building the house). After a stunned silence, the conversation quickly ended with me not doing the short. With hindsight, I wished the conversation had continued something like this: "But I don't know how to put up plaster board", "Good point", I would reply " I know how to do sound, so perhaps you should come and do more than the equivalent number of hours, to offset the skills difference."
Getting into the industry can be quite hard - the classic unbreakable cycle of unable to get a job without experience, but how do you get the experience without a job. There are apprenticeships out there, and trainee and entry level runner jobs, that do allow ways into the industry whilst earning a living. Working on a low budget productions may offer a way out of this cycle, but remember though, this is a job (albeit a very enjoyable one), and jobs are there to make us a living.