Returns are around 6-7% for good quality investments, but you really have to be very careful about where you invest and what type of property you purchase. Within the same district and often on the same street, there can be huge variations. There is very much an oversupply problem at the minute of low quality classic apartments, which nobody wants to buy/rent, but an undersupply and high demand for good quality properties at realistic prices.
If you're considering a new apartment, I'd recommend you research high-end developments such as Gozsdu Udvar or one of the more easily rentable developments in District XIII. Apartments in recently completed developments close to Gozsdu Udvar have also been trickling onto the market, but you should be aware too that after the competion of GU, there will be good availability of rental apartments in this part of VI/VII, so you will need to be realistic about your rental prospects and furnish well. In this area, it might be possible to find a nearly new apartment here for a more reasonable price. Prices are high for almost all new developments in the city. Gozsdu Udvar is one of the most prestigious, but at this stage, parking alone can cost €20,000-€24,000.
In new developments in the Angyalföld region of District XIII, the price per square metre for new builds is likely to be around €1,500. Rental returns should be good if you furnish well as it's a popular area for tenants, but IMO, there is probably not good long-term capital appreciation potential in this area. Tired of living on dirty streets in run-down parts of the city centre, some locals currently consider new buildings in this part of the city to be quite trendy to live in. However, this part of District XIII used to be one of the dodgiest areas of the city and the word 'Angyalföld' (Angel land) still has ironic connotations and is neither a particularly pleasant nor well-located part of the city. Most wealthier young Hungarians I know prefer to live in small apartments in prestigious parts of the downtown area (particularly the better parts of Districts I, V, VI). Younger (no children) expats I know personally only want to rent/buy in the downtown area.
I prefer to invest in renovated buildings in more prestigious areas, but it is almost impossible to find good apartments at reasonable prices in the best parts of the city. It typically takes a few months of searching. I'm always on the lookout, but rarely come across properties that I'd be happy to invest in.
If you choose to invest in classic buildings, you will find that the price/sqm will be a lot more realistic - at €1,200-€1,500 for renovated apartments in totally renovated buildings in premier locations. In my opinion, the capital appreciation potential for these properties should be higher. If you choose correctly and spend a bit extra on furnishing, you should be able to find foreign student/young professional tenants with little difficulty and achieve rental returns of 6-7%. Make sure to stay away from overpriced properties, buildings in poor condition, secondary locations, dark or noisy apartments, apartments with strange layouts, properties with a poor location within a building and overly busy streets. You'll find that this constitues 95-100% of properties currently on the market. For the most part, I prefer to stick to the quieter parts of District V, preferably not too close to Szent István Körút, where overpricing is more common or in District VI, particularly certain streets between Andrássy and Király, but close to the opera house. In my experience, renting high-quality classic apartments in this area at a reasonable price to expat students and young professionals has not been particularly difficult.
Budapest is a complicated market, so take your time and research extensively before committing to anything. Prices are unlikely to shoot up any time in the near future, so time is on your side.