This hobby requires significant time and dedication, especially if you're trying to do it on the cheap. I don't think it's something you can casually have a conversation on. Offer an hour of your time to walk through the mechanics and care and feeding of the system and inhabitants. At that time you can discuss pricing of individual components and alternative approaches. IMO, you either need to be very passionate about this hobby to get into it or plan to spend significant amounts of money on equipment and a management service. I don't think the casual idea of buying a tank for your kid is sufficient, especially if your initial concern is cost. I would probably convince them to take their kid to the fair and win them a goldfish or invest in Aquarium annual passes. Now, if you came to my house and was amazed by a Nessarius Snail crawling out of the sandbed after being disturbed by my sand sifting Diamond Goby and had to have a tank of your own then we can discuss options and share frags down the road.